Fix OpenStack tenant serializer registration: avoid double registration.
Fix OpenStack tenant serializer registration: avoid double registration.

opennode/waldur-mastermind,opennode/waldur-mastermind,opennode/waldur-mastermind,opennode/nodeconductor-assembly-waldur,opennode/waldur-mastermind,opennode/nodeconductor-assembly-waldur,opennode/nodeconductor-assembly-waldur