[SSSD] [sssd PR#957][opened] ci: add RHEL 7

[pbrezina]

   URL: https://github.com/SSSD/sssd/pull/957
Author: pbrezina
 Title: #957: ci: add RHEL 7
Action: opened

PR body:
"""
Add a RHEL 7 host to PR CI.
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/957/head:pr957
git checkout pr957