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
Add quota limit create/update methods to Quotas object

This adds two methods to the Quotas object that are effectively
passthroughs to the DB api calls. Since the Quotas object isn't
actually a representation of what is in the database, and because
we've (incorrectly) got some logic in the DB API around how the
quotas are applied, this is just a temporary step. Ideally, we
would (will) refactor Quotas in several ways in the near future,
at which time the way all of this works can be revisited and
cleaned up.

Related to blueprint compute-manager-objects-juno

Change-Id: If93614cd27e56667e0f106e8045dd1adc633ff1b
Merge "Add quota limit create/update methods to Quotas object"
Merge "Fix nova cells exiting on db failure at launch"