sp: setDomainRegularRole is implemented by backend

Commit ab6014b394 factored out metadata access to pool backend, but left
one call to StoragePool.setDomainRegularRole() that is implemented now
by the backend. Detected by pylint.

Change-Id: Ia49e45ccab0b933a8f1abc988ea0bd60e7b99b7d
Bug-Url: https://bugzilla.redhat.com/1096227
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24495
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>