Support for distro generation and many API cleanups

• Add dev-repo metadata to OPAM file.
• Add support for installing the cloud solver for platforms where aspcud is not available.
• Add CMD entrypoints for containers.
• Alpine: add bash in container (requested by @justincormack)
• Debian: correct non-interactive typos and add dialog in container
• Remove onbuild triggers from OPAM containers as it inhibits caching (suggestion via @talex5)
• Include specific Debian versions (v7,8,9) in addition to the stable/unstable streams.
• Add Dockerfile.crunch to reduce the number of layers by combining
  repeated RUN commands.
• Set Debian apt-get commands to noninteractive.
• Add support for Ubuntu 12.04 LTS and also bleeding edge 16.04.
• Add sexplib convertors for Dockerfile.t.
• Add Dockerfile_distro module to handle supported online distributions.
• Add Dockerfile.label to support Docker 1.6 metadata labels.
• Add generate_dockerfiles_in_git_branches to make it easier
  to use Docker Hub dynamic branch support to build all permutations.
• Correctly escape the run_exec, entrypoint_exec and cmd_exec
  JSON arrays so that the strings are quoted.
• Run yum clean after a Yum installation.
• Add support for Alpine Linux.
• Cleanup OPAM build directory to save container space after building from source.
• Remove support for OpenSUSE remotes, as it is no longer maintained.