IMPALA-2220: Fix flaky test test_create_drop_data_src
IMPALA-2220: Fix flaky test test_create_drop_data_src

Test was failing occasionally when the impalad returned by
ImpalaCluster.get_first_impalad() was not the same impalad
as that which the test class's self.client was connected to.
This now uses the ImpaladService created on the base class
to ensure it's the same impalad as the impalad that the
client is connected to.

Change-Id: I21799ba7a941b07f87f89dd19203bb8efc87ec11
Reviewed-on: http://gerrit.cloudera.org:8080/1699
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>

michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala