[wip][db/v1/instances] add replica_of field - trove database dbaas

[zhekazuev]

Issue

Fixes #2697

Description

• code: add ReplicaOf field in CreateOpts
• code: mark some values as a omiteptry and optional
• code: add non-empty checking for that field
• code: add checks for replicaOf field - that field cannot be specified with some other fields at the same time - TODO
• code: ...
  ...
• tests: add fixture for instance that created like a replica of different instance
• tests: ...
  ...
• tests: add link to api-ref samples - TODO
• tests: add more tests - TODO
• docs: TODO

Links

API:

• Service/InstanceController.create: https://github.com/openstack/trove/blob/40fdb7b44fb33e022b77ba8df63fde2565c70dea/trove/instance/service.py#L355
• Service/InstanceController.create checks: https://github.com/openstack/trove/blob/40fdb7b44fb33e022b77ba8df63fde2565c70dea/trove/instance/service.py#L368
• Change flavor as an optional parameter: openstack/python-troveclient@c23da58
• https://github.com/openstack/python-troveclient/blob/4e6e8cc73f504c5fd8c7d54acfb04932b9994680/troveclient/tests/fakes.py#L181
• https://github.com/openstack/trove-specs/blob/09b0eaace50c5307bf0a95a249e7515d1ba86ef5/specs/mitaka/pgsql-replication.rst#L52
• https://github.com/openstack/trove-dashboard/blob/75a36eac9384224911ceec66198716b14613fb96/trove_dashboard/api/trove.py#L152
• ...

Related:

• Trove: Add ability to create instance replicas via replica_of parameter #2698
• API Docs: https://docs.openstack.org/api-ref/database/?expanded=create-database-instance-detail#create-database-instance
• API Ref Parameters: https://github.com/openstack/trove/blob/40fdb7b44fb33e022b77ba8df63fde2565c70dea/api-ref/source/parameters.yaml#L799https://github.com/openstack/trove/blob/40fdb7b44fb33e022b77ba8df63fde2565c70dea/api-ref/source/parameters.yaml#L799
• https://github.com/piotrkukielka/gophercloud/tree/add-replica-of-parameret
• https://github.com/piotrkukielka/terraform-provider-openstack/tree/database-add-availabilty-zone-and-replica-of