Healing for EC2 inst/vol and S3 image mappings

Change-Id: Iccb2fdc9b4d2f7a12e4e5569ae6159de50ef5b79
Expire reservations in cells manager

Normally reservations are expired by a periodic task in nova-scheduler.
In a cells deployment quotas are usually enabled only on the api-level
cell. This uses a separate DB from the scheduler, so any reservations
on the top level never get expired.

This change adds the same periodic task to the cells manager so
reservations at all levels have a chance to be expired. The task is
disabled by default and can be turned on with the
CONF.cells.expire_reservations option.

Change-Id: I217f5b5e947662a858eb295f2add5c3227c3b531
Remove cell api overrides for lock and unlock

This isn't actually needed as these methods handle objects and is
triggered when the instance object is saved.
It actually caused a race condition with unlock as it triggers 2
instance updates from a compute cell.

Change-Id: I42eea6a2984f70527d4fc37f48ad973a2248721c
Closes-Bug: #1257168