Fix nova cells exiting on db failure at launch

We have seen cases where db errors at launch can cause cell
services to exit without retrying.  The service shouldn't
exit __init__ much like it doesn't later by handling
this type of exceptions.  We'll wait an hour and
then give up.

Change-Id: I24c9eb811d50d1fa6a5e4a5f595ebf68ded3b7b5
Closes-Bug: 1342257
Merge "Fix nova cells exiting on db failure at launch"