TestDrainingServer expects round robin region assignment but misses a config parameter

From https://builds.apache.org/view/G-L/view/HBase/job/HBase-TRUNK/2595/testReport/org.apache.hadoop.hbase/TestDrainingServer/org_apache_hadoop_hbase_TestDrainingServer/, we can see that some region server didn't have any regions assigned.

    // Assert that every regionserver has some regions on it.

It turns out that BulkEnabler has an internal knob whose value is false:

      boolean roundRobinAssignment = this.server.getConfiguration().getBoolean(
          "hbase.master.enabletable.roundrobin", false);

TestDrainingServer should have set this config parameter before calling admin.enableTable(TABLENAME)