Fix stopping instance in sync_power_states

When an instance is marked as stopped in the DB but is actually running,
the sync_power_states periodic task will try to stop it.  It does this
using the compute API.  The compute API checks the vm state before
proceeding.  The STOPPED state is treated as invalid.  This patch allows
the caller to bypass the state check and makes use of that in this
periodic task.

Change-Id: I447b6dbd1da72ed33067659708e1755bf8c933d0
Closes-bug: #1226965
Merge "Fix stopping instance in sync_power_states"