Migrate router flavors tests

Changes from original tests:
- Adjust migrated imports and configuration.
- Replaced test UUIDs with unique ones.
- Altered devstack ovn job for feature configuration.
- CLI prefix fetched using 'get_osp_cmd_prefix'.
- Adapted 'check_service_setting' parameters to whitebox method. 
- Migrated and adjusted 'cli_create_resource' base method
  for podified/devstack use.

Change-Id: I63c768b9ad352cb9809d94aa4743a7d5d00f0721