Cleanup: unneeded client definition, API override

- do not redefine a few variables for volume(admin and not) and backup clients
  when already defined in the base classes;
- define cls.admin_volume_client (same name as in tempest) rather then
  admin_volumes_client in the base class for admin volume tests;
- remove any usage and override of _api_version, which is not really used
  in the plugin, and it was also removed in tempest (see
  I6e05fbe21b29efa128fb59d16bf521d551b00304 )
  Even if unused, _api_version still pointed to the deprecated version 2
  for consistency group tests.

Change-Id: I3164662932a302b87ae2f346c0005bf7d0ab2927
4 files changed