The initial commit

Current status:

 * the agent supports cummunication with the server via oslo.messaging
 * the agent implements RPC interface required by
   https://review.openstack.org/#/c/71015/
   The interface and implementation is defined in
   savanna.guestagent.AgentEndpoint

Implements: blueprint guest-agent

Change-Id: I88cf5bb4c49b22293b0b299f182f55a4d179c877