Add CentOS7 CI config
Any reason why we weren't doing this before?
This commit will run tests for CentOS, and publish RPM packages on Gitlab Pages, see https://maxbes.ow2.io/lemonldap-ng/ for how the PPA will look like after merging this
I also had to fix one of the unit tests. It works on Debian because Cache::Memcached wasn't installed in the CI, but fails on CentOS because memcached isn't started by the test suite.