don't explode if a 413 didn't set Retry-After
don't explode if a 413 didn't set Retry-After

We use 413 error code not only for temporary out of memory
conditions but also for over quota issues. These kinds of
requests don't set a Retry-After header, but the code assumed
that was a valid key, thus exploding on a KeyError. Issue
caught in looking at tempest traces.

Fixes bug #1079387

Change-Id: Id406ab0799a35220b189b2e5292be092eb821ec5

paulmathews/nova,paulmathews/nova,paulmathews/nova