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 
What happens we intentionally kill an agent during shutdown?  I may be missing something, but wouldn't this also result in a non-zero exit status, but one that should be handled differently?