Clean IMAGE_SNAPSHOT_PENDING state on compute manager start up

There is a slim possibility that an instance will make it into the
state IMAGE_SNAPSHOT_PENDING  and get to the compute host just prior
to the stopping/failure of the compute service, this will result
in the instance being stuck in this state after the service resumes
and will prevent further actions on the instace by the user.

This change clears this state on start-up. Worst case it may result
in the request to snapshot being lost which will require the user to
re-request but I think this is preferable to leaving the instance in
a transitional state that the user cannot clear.

Related to blueprint recover-stuck-state

Change-Id: I8a4867f1f96d61e7d614d7d27c91f3d3e46f2400
Merge "Clean IMAGE_SNAPSHOT_PENDING state on compute manager start up"