Implement GET_SUMMARIES case in randomwalk security module

[DomGarguilo]

When running rwalk Security.xml the following line throws an exception. https://github.com/apache/accumulo-testing/blob/0a364eb2347775b2f8b7cd36408b3dc1faf9f2aa/src/main/java/org/apache/accumulo/testing/randomwalk/security/TableOp.java#L297-L298

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