Fix signature for launching int-tests

Reasons:
 - Trove-integration tests should have same
   signature to launch tests different datastores. Should be:
   - ./redstack int-tests --group={{datastore}}
   But MySQL has next signature:
   ./redstack int-tests --group=blackbox
 - To simplify gate template building Trove-integration int-tests
   need to have common shell execution format for
   any of integrated datastores.

Changes:
 - adding mysql group that equals to blackbox group;

Change-Id: I36aa311b83ebb2fc4b4d91b6c8d01af0c5412a8d
Closes-Bug: #1335270