Add restart policy in container configuration

#43 

Add restart policy in container configuration.
- always - always restart the container no matter of the exit code
- on-failure - restart the container only if has exited with non-zero exit code
- no - don't restart agent

@peterbourgon  @jmyounker  @stuartnelson3 
I might be wrong, but when we stop the container we send a signal, which means that the exitStatus will be 
agent.ContainerExitStatus{
		Signaled: true,
		Signal:   int(ws.Signal()),
} 
This case is still handled differently.