Reset task state 'migrating' on nova compute restart

This change has the compute manager's init_instance method check
for a task state set to 'migrating' and resets the task state to
None, so that live migrations that were pending when the compute
service went down do not result in instances that are perpetually
stuck in 'migrating' task state.

Change-Id: If23ea983cc048f317daae594f420a643f1bfe6ac
Closes-Bug: #1339823
Merge "Reset task state 'migrating' on nova compute restart"