qa/tasks/mgr: always add "application" label to pool

qa/tasks/mgr: always add "application" label to pool

otherwise monitor complains like
```
 health={'status': 'HEALTH_WARN', 'checks': [{'severity': 'HEALTH_WARN', 'summary': {'message': '1 pool(s) do not have an application enabled', 'count': 1}, 'detail': [{'message': "application not
enabled on pool 'dashboard_pool3'"}, {'message': "use 'ceph osd pool application enable <pool-name> <app-name>', where <app-name> is 'cephfs', 'rbd', 'rgw', or freeform for custom applications."}],
'muted': False, 'type': 'POOL_APP_NOT_ENABLED'}], 'mutes': []}
```

and if test checks for the healthy status, the test fails like:
```
2020-04-30T17:29:54.498 INFO:tasks.cephfs_test_runner:    self.assertEqual(health['status'], 'HEALTH_OK', msg='health={}'.format(health))
2020-04-30T17:29:54.498 INFO:tasks.cephfs_test_runner:AssertionError: 'HEALTH_WARN' != 'HEALTH_OK'
```

Signed-off-by: Kefu Chai <kchai@redhat.com>