IMPALA-3491: Use unique_database fixture in test_last_ddl_time_update.py.
IMPALA-3491: Use unique_database fixture in test_last_ddl_time_update.py.

Testing: Ran the test locally in a loop 10 times, and did an
exhaustive private test run on HDFS.

Change-Id: I97e96217301078d48584c51218345dc96f6853a6
Reviewed-on: http://gerrit.cloudera.org:8080/3104
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>

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