Port run-tox.sh change in to run-cover.sh

cover can run on CentOS nodes, so we need this here too.

Change-Id: I6bc4a69559ecd90f12700910c1438962690e9e54
Reviewed-on: https://review.openstack.org/29685
Reviewed-by: matthew wagoner <zxkuqyb@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins