Move nova_utils functions to nova client plugin

All uses of nova_utils functions have been migrated to using
the nova client plugin version.

Existing functions now have a deprecation warning which is
suppressed during nova_utils unit tests.

nova_utils.get_flavor_id was being called with the trove
client, so there is now a dedicated trove plugin method
get_flavor_id.

Change-Id: Ic8120022a1f9117c783f6422069f94614bcbb3c6