Monitoring (MaaS) support

[tintoy]

Where practical, retain the semantics of the API as CloudControl provides it (rather than trying to provide an abstraction / application-level error handling as part of the client).

• Expose monitoring status on Server struct.

APIs:

• Server monitoring
  • Enable monitoring for a Server
  • Disable monitoring for a Server
  • Change server monitoring plan

Additionally:

• Unit tests
• Integration tests
• GoDoc comments