Optimize service find in migration
Optimize service find in migration

Run only once will be better of volume_utils.extract_host(host, 'backend'),
It's a waste time in every iteration.

Change-Id: I161de42b8ea421766f1fd980ac64334ca9f1c5fd

cloudbase/cinder,phenoxim/cinder,Hybrid-Cloud/cinder,Datera/cinder,j-griffith/cinder,mahak/cinder,mahak/cinder,NetApp/cinder,j-griffith/cinder,phenoxim/cinder,eharney/cinder,eharney/cinder,Nexenta/cinder,Hybrid-Cloud/cinder,Datera/cinder,bswartz/cinder,NetApp/cinder,openstack/cinder,ge0rgi/cinder,cloudbase/cinder,bswartz/cinder,Nexenta/cinder,openstack/cinder