[opam-devel] bulk testing now includes CentOS 6/7

[Anil Madhavapeddy]

At a request from Dave Scott from Citrix, I've added a Docker CentOS 6/7 image into https://github.com/avsm/docker-opam and started running bulk builds on there too.

This looks for tags of the form ["centos"] and/or ["centos" "centos-7"] (for more
specific RPM versions).  Results are being pushed to https://github.com/ocaml/opam-bulk-logs, and I anticipate there will be many more failures due to the lack of depexts for this platform.

Dave has made a start with https://github.com/ocaml/opam-repository/pull/2656, but any other interested parties in CentOS/Fedora can also contribute depexts.

cheers,
Anil