132fbcd38ebae52bdd20da54905131b75581520f in cinder changed the
volume_clear StrOpt to use the choices kwarg which enforces the value
specified and raises a ValueError if an invalid value is set for the
option in cinder.conf.
This lets us remove the validation that devstack was doing.
Change-Id: Ia7eead6297ed0f3a972de2021170fe9c7225e856