Add a CI runner for CentOS 7 + EPEL 7
With EPEL repos enabled, both cerbero and gst-build should be usable on CentOS 7. RHEL 7 / CentOS 7 is a common deployment target, but none of us use it as a development OS, so it breaks quite easily.
If we run into a lot of issues getting, f.ex., validate to pass, we can skip that and do a compile-only CI for EPEL 7.
CC: @meh