test_alarm_redirect_keystone may fail with devstack

I was testing code with devstack master for ceilometer minimum installation, devstack has configured (according to my local.conf) /etc/ceilometer/ceilometer.conf with [DEFAULT] dispatcher = gnocchi

Then when I tried to run tox -efunctional, the test_alarm_redirect_keystone failed:

     WARNING [ceilometer.api.controllers.v2.root] ceilometer-api started with aodh enabled. Alarms URLs will be redirected to aodh endpoint.
     WARNING [ceilometer.api.controllers.v2.root] ceilometer-api started with gnocchi enabled. The resources/meters/samples URLs are disabled.

    Traceback (most recent call last):
      File "ceilometer/tests/functional/api/v2/test_api_upgrade.py", line 115, in test_alarm_redirect_keystone
        self.ks.service_catalog.url_for.mock_calls)
      File "/opt/stack/ceilometer/.tox/functional/local/lib/python2.7/site-packages/testtools/testcase.py", line 350, in assertEqual
        self.assertThat(observed, matcher, message)
      File "/opt/stack/ceilometer/.tox/functional/local/lib/python2.7/site-packages/testtools/testcase.py", line 435, in assertThat
        raise mismatch_error
    testtools.matchers._impl.MismatchError: !=:
    reference = [call(service_type='alarming')]
    actual = [call(service_type='alarming'), call(service_type='metric')]

So I was bitten by /etc/ceilometer/ceilometer.conf AGAIN.

The minimum change is we override dispatcher option before that test is executed