Add posargs to tox coverage job.

This commit adds the posargs field to the tempest_coverage report
command on the tox coverage job. This will allow us to pass in
arguments to the report command which will be useful for the
periodic coverage runs so we can specify the output directory.

Change-Id: I89c9c8da7fbe5f73600a8e84557dbf325c341afc
Merge "Add posargs to tox coverage job."