Retry the vmsctl discard command on errors

OS-266

Immediately after migrating an launched instance there is some
cool-down period where the blessed instance cannot be discarded.
This updates cobalt to do a better-effort to discard a blessed
instance by retrying a couple times.

Change-Id: I6e8598ca6907f2d3af73e09b42b6575cb79b13a5
Merge 'Retry vmsctl discard on errors (OS-266)' into havana/1.5

Conflicts:
	cobalt/nova/extension/manager.py

Change-Id: Ic4820ba63b0fa80179fe4077c840caee671bf322