Integrate MyRocks MTR tests 5.6

-Add RocksDB plugin to MTR plugin.defs to allow auto loading and setting of variables $ROCKSDB_OPT $ROCKSDB_LOAD_ADD.
-Add $ROCKSDB_OPT $ROCKSDB_LOAD_ADD to RocksDB suite.opt files.
-Fixup/implement have_rocksdb.inc and have_rocksdb_as_default.inc and touch tests to use proper have_* file.

5.6
http://jenkins.percona.com/view/PS%205.6/job/percona-server-5.6-param/1451/
https://github.com/percona/percona-server/pull/1105