Closes-Bug: #13729271
Issue: "instance" arg is not passed to imagebackend snapshot.
Fix: "instance" arg is passed from compute manager.

Change-Id: I5b9dc33f296066a6c09ef709a8099fbd4dd41ae8
Merge "Closes-Bug: #13729271 Issue: "instance" arg is not passed to imagebackend snapshot. Fix: "instance" arg is passed from compute manager." into contrail/icehouse