Feature/rest client

Created interfaces for Coopr Rest Client API. @albertshau , please take a look. Interfaces contain all methods, are listed in the ticket https://issues.cask.co/browse/COOPR-493.  

**Note:** I added dependency to the *coopr-server* project in to the *coopr-rest-client* ```pom.xml``` for have ability to use some complicated request classes, like ```ClusterCreateRequest```. So, I think, it would be good if we could move these request objects to the separate module, which will be accessible for all other parts of Coopr system. It could be *coopr-common* module, or something like this. Please, let us to know, what do you think about this proposal.  
Javadoc should describe what is returned if no such template exists. Assume the plan is to return null?