Feature/rest client

Added unit tests for negative cases in the Cluster Client API. Created README.md file with documentation for the Coopr REST Client API. And refactored ```AdminRestClient``` according to Albert's comments in the previous PR. @albertshau , please, take a look.
Handler will send BadRequest response, if cluster name is unexpected. It is just for emulate some unexpected action. Rest client doesn't create ```ClusterCreateRequest``` object, so, I think there are no reason to check here is the ```numMachines``` value correct or not. Here I just test, how client could behave, if it will receive unsuccessful response. But if request object should be validated in the client side, please let me to know.