Improve logging for volume detach
Improve logging for volume detach

Currently the volume manager doesn't log the instance ID during
volume detach. This is missing in cinder-api log as well. This
patch adds a log statement to print a message with volume ID and
instance ID before calling driver detach.

Change-Id: I6e765124c4e737e64cff39ec82cb81749bca28ab

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