Find free device during host startup.

This is Folsom backport

In some cases original loop device is busy before it is losetup-ed to serve for VG. Added logic to find free one if this happens.
Fixes: rhbz#971145, #947624

Conflicts:
	packstack/plugins/cinder_250.py

Change-Id: I2fd357a9a6365f4b082bcf6b03a57ff3d5d429a8
Restart openstack-cinder-volume service

Change-Id: If675261076501557f1063e33163fc0e322feb5eb
Fixes: rhbz#975007