Alpine 3.5, Ubuntu 16.10 and remote solver improvements

• Remove support for ARM variants from the default distribution
  list. They will come back as explicitly supported multiarch
  targets, instead of the current qemu builds that are mixed up
  with x86_64 targets.
• Always install OPAM from source on Alpine until upstreaming
  is complete.
• Register 4.04 as a mainline compiler as well (fixes OPAM2).
• Add support for Alpine 3.5 and OpenSUSE 42.2, and promote
  the Alpine:latest images to Alpine 3.5.
• Do not install camlp4 by default in distributions.
• Refresh aspcud remote proxy with url-escaping fixes
  (via @OCamlPro-Henry in ocaml/opam#2809)
• Add Ubuntu 16.10 to the built-distros list.