instance state will be stuck in unshelving when unshelve fails

When unshelve an instance, if this instance has been offloaded,
the conductor manager will involve re-schedule for the instance.
If re-schedule failed to find a target host for the unsleved
instance, then the instance will be stuck in unshelving state.

This patch fix the issue as this: If re-schedule failed to find a
target host for unshelve the instance, then conductor manager will
try to rollback the instance to unshelve state.

Change-Id: If49b2d2c9263b853c745ce25fe146ade51948123
Closes-Bug: #1248799
Merge "instance state will be stuck in unshelving when unshelve fails"