IMPALA-13929: Make 'functional-query' the default workload in tests

This change adds get_workload() to ImpalaTestSuite and removes it
from all test suites that already returned 'functional-query'.
get_workload() is also removed from CustomClusterTestSuite which
used to return 'tpch'.

All other changes besides impala_test_suite.py and
custom_cluster_test_suite.py are just mass removals of
get_workload() functions.

The behavior is only changed in custom cluster tests that didn't
override get_workload(). By returning 'functional-query' instead
of 'tpch', exploration_strategy() will no longer return 'core' in
'exhaustive' test runs. See IMPALA-3947 on why workload affected
exploration_strategy. An example for affected test is
TestCatalogHMSFailures which was skipped both in core and exhaustive
runs before this change.

get_workload() functions that return a different workload than
'functional-query' are not changed - it is possible that some of
these also don't handle exploration_strategy() as expected, but
individually checking these tests is out of scope in this patch.

Change-Id: I9ec6c41ffb3a30e1ea2de773626d1485c69fe115
Reviewed-on: http://gerrit.cloudera.org:8080/22726
Reviewed-by: Riza Suminto <riza.suminto@cloudera.com>
Reviewed-by: Daniel Becker <daniel.becker@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

tests/custom_cluster/test_workload_mgmt_init.py

@@ -49,10 +49,6 @@ class TestWorkloadManagementInitBase(CustomClusterTestSuite):
   This test class does not extend WorkloadManagementTestSuite because its subclasses
   define its own setup_method and teardown_method."""
 
-  @classmethod
-  def get_workload(self):
-    return 'functional-query'
-
   @classmethod
   def default_test_protocol(cls):
     return HS2