Implement GET_SUMMARIES case in randomwalk security module

[DomGarguilo]

When running rwalk Security.xml the following line throws an exception.

accumulo-testing/src/main/java/org/apache/accumulo/testing/randomwalk/security/TableOp.java

Lines 297 to 298 in 0a364eb

	case GET_SUMMARIES:
	throw new UnsupportedOperationException("GET_SUMMARIES not implemented");

I think it would be pretty easy to implement this so the rwalk security module could run properly again.