Intial Agent remote implementation
Intial Agent remote implementation

So, here is the start for Guest Agent support. Savanna communicates with
the agent via oslo.messaging. The agent's code will be committed later in
a separate repository.

One remark: current code assumes that the agent is not pre-installed on
the image, so additional userdata is added. The userdata installs
and starts the agent.

Current limitations:
  * Works for Vanilla and HDP plugins
  * Works on Ubuntu and CentOS guests
  * Works with Rabbit MQ only

Note: the guest agent repo is at
https://github.com/stackforge/savanna-guestagent

Correspondingly the project name in gerrit is stackforge/savanna-guestagent

Implements: blueprint guest-agent
Change-Id: I787b6b058989568553815bc6569e6fe8f46df6b5

openstack/sahara-tests,esikachev/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,esikachev/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,esikachev/sahara-tests