juwang-enable virtual core in Java Reef and clr bridge

Currently Reef clients cannot set virtual core when they request evaluators. This PR is to enable the virtual core in EvaluatorRequest so that the clients can set core count when an EvaluatorRequest is created.  
The request is submitted through EvaluatorRequstor and eventually reaches to ResourceRequestHanlder and set to Resource object. ResourceRequestProto schema is updated for this.
After a Container is allocated, the core data is passed back from YARN. ResourceAllocationProto and EvaluatorDescriptor are also modified to hold the data. 
Submit() in EvaluatorRequestorBridge is updated to add this parameter. All clr bridge related methods are also modified accordingly. 

This is for #995

Please fix the formatting and use the plural `virtual_cores`