As 284, part 2 - check for server on nova error

This part actually does the create server retries after checking to make sure no server was created.  This is based on https://github.com/rackerlabs/otter/pull/647
Hm. Does Nova document exactly what kind of regexes they use here? (Sorry, ISTR already asking this, and it may have already been answered). I'm considering differences between e.g. PCRE and Python regexen.