unit test search unnecessary directories for test cases

[Can Zhang]

Currently in `.testr.conf`, the test_command is written as `${PYTHON:-python} -m subunit.run discover ceilometer -t . $LISTOPT $IDOPTION`, which actually searches `ceilometer/ceilometer/*`. But as all unit tests lie in `ceilometer/ceilometer/tests`, the test_command should use `ceilometer/tests` instead of `ceilometer`.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/120969

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/120969
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=dd772619e8722612a68f0b27787e6cc662e7654a
Submitter: Jenkins
Branch: master

commit dd772619e8722612a68f0b27787e6cc662e7654a
Author: Can ZHANG <email address hidden>
Date: Fri Sep 12 12:01:11 2014 +0800

    Eliminate unnecessary search for test cases

    As all unit tests lie in `ceilometer/ceilometer/tests`,
    the test_command should use `ceilometer/tests` instead of `ceilometer`.

    Change-Id: Idb1dade44a06c71fb5e64ca875b71633def110a5
    Closes-Bug: 1368509