Integration test for JDBC driver in Grill server

Changes - 
1. Added JDBC test case as sub class of TestQueryService
2. Changed most validate* methods in TestQueryService from static to instance methods so that they can be overridden in JDBC test case
3. Made driver map in QueryExecutionServiceImpl protected so that it can be manipulated in tests
4. Fixed VARCHAR datatype in JDBC result set.
Adding and removing JDBCDriver for each test is too much intrusive. Can we start query service with both HIveDriver and JDBCDriver for tests, by specifying the drivers config? Current queries should still work because the driver pickup logic should work.