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 
`"%q should be %s, %s, or %s"`