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