Unit test of multiple container requests
 * Added `YarnResourceRequestHandlerTest` to test `YarnResourceRequestHandler`
 * Added more logging to `YarnContainerManager.onContainerRequest()`
 * Extracted the interface of `YarnContainerRequestHandler` and moved its implementation to `YarnContainerRequestHandlerImpl`
Merge pull request #778 from Microsoft-CISL/weimer_774

Unit test of multiple container requests