Reduce the number of REST clients in use

Maintain a "worker context" for each thread with REST clients for nova
and neutron, and pass them to the Automaton and VmManager classes when
a thread needs to do some work with them.

Change-Id: I3d286969b73eacfead2d748415bb060dd78e9e09
Merge pull request #96 from dhellmann/feature/share-rest-client-handles

Reduce the number of REST clients in use