Enable multi-threaded nova API servers

Add an additional worker thread for each nova service: compute, ec2 and
metadata.  Any real deployment will be using multiple API server
threads, so lets make devstack test that scenario.  With this patch we
will have four worker threads for each type of API server.

Change-Id: I09f4c6f57e71982b8c7fc92645b3ebec12ff1348
Merge "Enable multi-threaded nova API servers"