[RDSv3] Implement description management

[anton-sidelnikov]

Need to implement: https://docs.otc.t-systems.com/relational-database-service/api-ref/api_v3_recommended/db_instance_management/changing_the_description_of_a_db_instance.html

Add:
openstack/rds/v3/instances/UpgradeDescription.go
And update test:
acceptance/openstack/rds/v3/rds_test.go

[bakhterets]

This method is not yet supported by the API.
error: The API does not exist or has not been published in the environment