Make sure multiple test base classes work for Explore

PR for https://issues.cask.co/browse/CDAP-729
In a unit-test context, if two test classes both extending TestBase have record scannable datasets, the initialization of the Hive Metastore for the second test class will not have the default databases created - for some reason. The quick fix is to manually recreate them at Explore start-up.
Build: https://builds.cask.co/browse/CDAP-RBT15
This seems to be a band-aid fix. We should look at the underlying issue that work around in the explore service. At the very least we should know why the DBs are not created for the second test.