OPAM2 support, OpenSUSE/Ubuntu and distro refresh

• Multiarch: Add Alpine 3.4 and Alpine/ARMHF 3.4 and deprecate Raspbian 7.
• Add OpenSUSE/Zypper support and add OpenSUSE 42.1 to the default distro build list.
• Add Ubuntu 16.10 to the distro list, and remove Ubuntu 15.10 from default build list now that 16.10 LTS is available.
• Add Fedora 24 and make it the alias for Fedora stable. Also install redhat-rpm-config which is needed for pthreads.
• Add an extra arg the Dockerfile_distro matrix targets to add more distros to the mix, such as Raspbian.
• Support multiple OPAM versions in the matrix generation, to make testing OPAM master easier.
• Always do an rpm --rebuilddb before a Yum invocation to deal with possible OverlayFS brokenness.
• Support opam_version to distro calls to build and install the latest version of OPAM2-dev.
• Add xz into Alpine containers so that untar of those works.
• Expose the development versions of OCaml compilers.