db/v1/instance: Add configuration to createOpts

[nikParasyr]

Add configuration group id to createOpts and update unit tests accordingly.

Docs: https://docs.openstack.org/api-ref/database/#create-database-instance

[coveralls]

coverage: 77.906% (+0.002%) from 77.904%
when pulling 086cd1b on nikParasyr:db_instance_config
into d8db226 on gophercloud:master.

[pierreprinetti]

@EmilienM Do you plan on backporting this patch to a release?