Add familyPrefix option to ecs:ListTaskDefinitions

AWS defines this option as:
```
--family-prefix (string)
  The full family name with which to filter the ListTaskDefinitions
  results. Specifying a familyPrefix limits the listed task
  defini-tions to task definition revisions that belong to that
  family.
```

This option behaves differently than ecs:ListTaskDefinitionFamilies.
Instead of doing a comparison like `startswith`, it does a full string
comparison by matching the entire task definition family to the prefix.
For example, let's say there exists a task definition with the family
`super-cool-task-def`.

ListTaskDefinitionFamilies would look like this:

```
aws ecs list-task-definition-families --family-prefix super-cool
{
    "families": [
      "super-cool-task-def"
    ]
}
```

ListTaskDefinitions would look like this:

```
aws ecs list-task-definitions --family-prefix super-cool
{
    "taskDefinitionArns": []
}
```

moto/ecs/models.py

@@ -567,16 +567,14 @@ class EC2ContainerServiceBackend(BaseBackend):
 
         return task_definition
 
-    def list_task_definitions(self):
+    def list_task_definitions(self, family_prefix):
-        """
-        Filtering not implemented
-        """
         task_arns = []
         for task_definition_list in self.task_definitions.values():
             task_arns.extend(
                 [
                     task_definition.arn
                     for task_definition in task_definition_list.values()
+                    if family_prefix is None or task_definition.family == family_prefix
                 ]
             )
         return task_arns

moto/ecs/responses.py

@@ -68,7 +68,8 @@ class EC2ContainerServiceResponse(BaseResponse):
         return json.dumps({"taskDefinition": task_definition.response_object})
 
     def list_task_definitions(self):
-        task_definition_arns = self.ecs_backend.list_task_definitions()
+        family_prefix = self._get_param("familyPrefix")
+        task_definition_arns = self.ecs_backend.list_task_definitions(family_prefix)
         return json.dumps(
             {
                 "taskDefinitionArns": task_definition_arns

tests/test_ecs/test_ecs_boto3.py

@@ -171,6 +171,69 @@ def test_list_task_definitions():
     )
 
 
+@mock_ecs
+def test_list_task_definitions_with_family_prefix():
+    client = boto3.client("ecs", region_name="us-east-1")
+    _ = client.register_task_definition(
+        family="test_ecs_task_a",
+        containerDefinitions=[
+            {
+                "name": "hello_world",
+                "image": "docker/hello-world:latest",
+                "cpu": 1024,
+                "memory": 400,
+                "essential": True,
+                "environment": [
+                    {"name": "AWS_ACCESS_KEY_ID", "value": "SOME_ACCESS_KEY"}
+                ],
+                "logConfiguration": {"logDriver": "json-file"},
+            }
+        ],
+    )
+    _ = client.register_task_definition(
+        family="test_ecs_task_a",
+        containerDefinitions=[
+            {
+                "name": "hello_world",
+                "image": "docker/hello-world:latest",
+                "cpu": 1024,
+                "memory": 400,
+                "essential": True,
+                "environment": [
+                    {"name": "AWS_ACCESS_KEY_ID", "value": "SOME_ACCESS_KEY"}
+                ],
+                "logConfiguration": {"logDriver": "json-file"},
+            }
+        ],
+    )
+    _ = client.register_task_definition(
+        family="test_ecs_task_b",
+        containerDefinitions=[
+            {
+                "name": "hello_world2",
+                "image": "docker/hello-world2:latest",
+                "cpu": 1024,
+                "memory": 400,
+                "essential": True,
+                "environment": [
+                    {"name": "AWS_ACCESS_KEY_ID", "value": "SOME_ACCESS_KEY2"}
+                ],
+                "logConfiguration": {"logDriver": "json-file"},
+            }
+        ],
+    )
+    empty_response = client.list_task_definitions(familyPrefix="test_ecs_task")
+    len(empty_response["taskDefinitionArns"]).should.equal(0)
+    filtered_response = client.list_task_definitions(familyPrefix="test_ecs_task_a")
+    len(filtered_response["taskDefinitionArns"]).should.equal(2)
+    filtered_response["taskDefinitionArns"][0].should.equal(
+        "arn:aws:ecs:us-east-1:012345678910:task-definition/test_ecs_task_a:1"
+    )
+    filtered_response["taskDefinitionArns"][1].should.equal(
+        "arn:aws:ecs:us-east-1:012345678910:task-definition/test_ecs_task_a:2"
+    )
+
+
 @mock_ecs
 def test_describe_task_definition():
     client = boto3.client("ecs", region_name="us-east-1")
@@ -1756,7 +1819,7 @@ def test_update_task_definition_family_through_cloudformation_should_trigger_a_r
     cfn_conn.update_stack(StackName="test_stack", TemplateBody=template2_json)
 
     ecs_conn = boto3.client("ecs", region_name="us-west-1")
-    resp = ecs_conn.list_task_definitions(familyPrefix="testTaskDefinition")
+    resp = ecs_conn.list_task_definitions(familyPrefix="testTaskDefinition2")
     len(resp["taskDefinitionArns"]).should.equal(1)
     resp["taskDefinitionArns"][0].endswith("testTaskDefinition2:1").should.be.true