rds: AvailabilityZone is not printed out in describe_db_instances

[t3hG04T]

Despite being taken as an argument for create_db_instance (and stored in the Database model), the AvailabilityZone (DBInstance/AvailabilityZone) is not printed out in the:

rds.describe_db_instances(DBInstanceIdentifier='arn')

call.

Actual response in boto3:

• create_db_instance
• describe_db_instances

This is not a problem for describe_db_clusters.

The description of this report is sparse, as I would like to submit a PR right away (this boils down to adding one line in to_xml of the Database model).