Added explicit Evaluator memory size requests to YARN

Closes #674: The YARN runtime now supports Evaluator requests with explicit memory size. In fact, if such an explicit memory size is given, it take precedence over the old `S,M,L,XL` sizes.

Also, this deprecates the old sizes in the hope of removing them in 0.3.

@motus Please verify this on an actual YARN cluster before merging. Concretely, `com.microsoft.reef.tests.evaluatorsize.EvaluatorSizeTest` needs to pass before we can merge this and consider #674 closed.