Make ironic-api not single threaded

The ironic-api service is an instance of wsgiref
simple_server.WSGIServer.  This derives from SocketServer.TCPServer,
which requires that SocketServer.ThreadingMixIn be mixed in for it to
become thread'able, however this was not being done when starting the
WSGIServer.

This patch creates a ThreadedSimpleServer class, which is just a mix
of ThreadingMixIn and WSGIServer, so that the ironic-api process
does not block on network IO.

Change-Id: I9d686765edb4975519840a51d3beeb2be797a4fa
Related-bug: #1279206
Merge "Make ironic-api not single threaded"