Mock Mesos Server for testing

[chetanmeh]

Current implementation is decoupled from HTTP semantics and hence makes it possible to have test coverage for offer handling logic. However it would be nice to have a mock server which can enable testing of HTTP parts.

This can be implemented as a simple Akka HTTP Server. For an example see MesosServerSimulation used in mesos rx-java.

[chetanmeh]

Currently in our code while making HTTP calls we do not consume the response in error case and some other cases. So having HTTP test would be helpful here