fix(rds2): handle create_db_instance when AllocatedStorage is not specified

In all of the tests of `create_db_instance()`, the `AllocatedStorage`
parameter is provided.  The [RDS API
reference](https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBInstance.html)
says this parameter is optional; however, when none is provided, moto
returns an obscure error message:

```
self = <botocore.parsers.QueryParser object at 0x113745890>, shape = <Shape(Integer)>, text = 'None'

    @_text_content
    def _handle_integer(self, shape, text):
>       return int(text)
E       ValueError: invalid literal for int() with base 10: 'None'

/usr/local/Cellar/pyenv/1.2.13_1/versions/3.7.4/envs/rds_encrypt/lib/python3.7/site-packages/botocore/parsers.py:466: ValueError
```

This PR adds default values that correspond to the current default API
behaviors.

moto/rds2/models.py

@@ -56,6 +56,8 @@ class Database(BaseModel):
         else:
             self.kms_key_id = kwargs.get("kms_key_id")
         self.storage_type = kwargs.get("storage_type")
+        if self.storage_type is None:
+            self.storage_type = Database.default_storage_type(iops=self.iops)
         self.master_username = kwargs.get('master_username')
         self.master_user_password = kwargs.get('master_user_password')
         self.auto_minor_version_upgrade = kwargs.get(
@@ -63,6 +65,8 @@ class Database(BaseModel):
         if self.auto_minor_version_upgrade is None:
             self.auto_minor_version_upgrade = True
         self.allocated_storage = kwargs.get('allocated_storage')
+        if self.allocated_storage is None:
+            self.allocated_storage = Database.default_allocated_storage(engine=self.engine,storage_type=self.storage_type)
         self.db_instance_identifier = kwargs.get('db_instance_identifier')
         self.source_db_identifier = kwargs.get("source_db_identifier")
         self.db_instance_class = kwargs.get('db_instance_class')
@@ -292,6 +296,79 @@ class Database(BaseModel):
             'sqlserver-web': 1433,
         }[engine]
 
+    @staticmethod
+    def default_storage_type(iops):
+        if iops is None:
+            return 'gp2'
+        else:
+            return 'io1'
+
+
+    @staticmethod
+    def default_allocated_storage(engine,storage_type):
+        return {
+            'aurora': {
+                'gp2': 0,
+                'io1': 0,
+                'standard': 0,
+            },
+            'mysql': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 5,
+            },
+            'mariadb': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 5,
+            },
+            'postgres': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 5,
+            },
+            'oracle-ee': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 10,
+            },
+            'oracle-se2': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 10,
+            },
+            'oracle-se1': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 10,
+            },
+            'oracle-se': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 10,
+            },
+            'sqlserver-ee': {
+                'gp2': 200,
+                'io1': 200,
+                'standard': 200,
+            },
+            'sqlserver-ex': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 20,
+            },
+            'sqlserver-se': {
+                'gp2': 200,
+                'io1': 200,
+                'standard': 200,
+            },
+            'sqlserver-web': {
+                'gp2': 20,
+                'io1': 100,
+                'standard': 20,
+            },
+        }[engine][storage_type]
+
     @classmethod
     def create_from_cloudformation_json(cls, resource_name, cloudformation_json, region_name):
         properties = cloudformation_json['Properties']

moto/rds2/responses.py

@@ -42,7 +42,7 @@ class RDS2Response(BaseResponse):
             "region": self.region,
             "security_groups": self._get_multi_param('DBSecurityGroups.DBSecurityGroupName'),
             "storage_encrypted": self._get_param("StorageEncrypted"),
-            "storage_type": self._get_param("StorageType", 'standard'),
+            "storage_type": self._get_param("StorageType", None),
             "vpc_security_group_ids": self._get_multi_param("VpcSecurityGroupIds.VpcSecurityGroupId"),
             "tags": list(),
         }

tests/test_rds2/test_rds2.py

@@ -39,6 +39,20 @@ def test_create_database():
     db_instance['VpcSecurityGroups'][0]['VpcSecurityGroupId'].should.equal('sg-123456')
 
 
+@mock_rds2
+def test_create_database_no_allocated_storage():
+    conn = boto3.client('rds', region_name='us-west-2')
+    database = conn.create_db_instance(
+        DBInstanceIdentifier='db-master-1',
+        Engine='postgres',
+        DBName='staging-postgres',
+        DBInstanceClass='db.m1.small')
+    db_instance = database['DBInstance']
+    db_instance['Engine'].should.equal('postgres')
+    db_instance['StorageType'].should.equal('gp2')
+    db_instance['AllocatedStorage'].should.equal(20)
+
+
 @mock_rds2
 def test_create_database_non_existing_option_group():
     conn = boto3.client('rds', region_name='us-west-2')