Fix tox coverage job

To run the tempest unit tests with coverage enabled the coverage
module needs to be installed. This requirement is properly listed in
test-requirements.txt, however the coverage job definition wasn't
installing coverage. This commit fixes this oversight by adding the
test-requirements file as a dependency for the job.

Change-Id: Ic443c6c4a3166e6b198cbe2af0a77ff2f0a38e83
Merge "Fix tox coverage job"
Merge branch 'master' of https://github.com/openstack/tempest