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.
7b1cf9eecd