Add a command and tox environment to let us define a job to ensure that
all repos in projects.txt have the check-requirements job defined. Two
projects are missing the job now, so this patch depends on the patch to
add those jobs.
Change-Id: I61658a7265a77e72401978dda558f41b28a81d92
Depends-On: I81e1c70257e28583e776171580d75341e620bf8d
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
