The lower-constraints test was removed becuase of an issue where pip
could not correctly determine the required packages versions to install,
ending in an almost infinite loop that would end up in timeout, failure,
and general mayhem.
Recently the issue has been fixed and, if properly configured, the
lower-constraints test can provide good indication of which minimum
versions are required to support the current code.
This patch adds the test back to the current development branch.
The long term goal is to keep the lower-constraints file in the stable
branches, but remove the test job to avoid issues during backports.
Change-Id: I5fff32ec5dd1a045116bcf02349650b1f5e3a196
e5d35ba65c