Allow to specify multiple tests in run_tests.sh

[OpenStack Infra]

https://review.openstack.org/155748
commit 6270ce08c7c3729f87eccf1f2d41e7e12250b5ee
Author: Bartłomiej Piotrowski <email address hidden>
Date: Fri Feb 13 14:53:49 2015 +0100

    Allow to specify multiple tests in run_tests.sh

    Disable checking for unbound variables, as there is possibility that no
    tests were specified. Example invocation:

    run_tests.sh -n -t fuelclient/tests/test_client.py:TestAuthentication.test_wrong_credentials \
    -t fuelclient/tests/test_client.py:TestDownloadUploadNodeAttributes

    Additionally align shifts in getopt loop and remove
    dead return from tox invocation.

    DocImpact
    Change-Id: I83a52b5f5467c9ea92dfafba9b92b700f7e76a38
    Closes-Bug: 1420664