Update service client access in tempest tests

All the Designate service clients are registered via the plugin interface[1],
that way Tempest register and create the lazy initialization of registered
clients so that they can be access from there in consistent way.

But designate-tempest-client create a separate instance of those and access
instead of accessing the registered service client in Tempest. This commit
makes all the service clients access from Tempest registry and remove the
separate objects.

[1] https://github.com/openstack/designate-tempest-plugin/blob/master/designate_tempest_plugin/plugin.py#L78

Change-Id: I4b65ca16cd53ff4c6bece642764a4abe2a469c01
27 files changed