xapi backport: use OCaml 4.02.3, PPX, xs-opam

This commit is part of a larger refactoring that is backported from
branch master:

* Update the OCaml compiler to version 4.02.3.

* Avoid using Camlp4 and use the PPX macro system instead.

* Upgrade OCaml upstream libraries and move away in those libraries from
  using OCamlp4 in favour of PPX.

* Updated upstream OCaml libraries are bundled into a two new packages
  xs-opam-src and xs-opam-repo. These provide an Opam environment
  holding OCaml libraries. This means that many OCaml libraries are no
  longer provided in individual RPM packages but in xs-opam-repo.

* The Baggage RPM repository is dropped from the build environment. It
  contained the OCaml compiler, some OCaml upstream libraries, and some
  other packages. These are either incorporated into xs-opam or into new
  RPM packages part of this branch.

* RPM packages for packages that are now part of xs-opam-{src.repo} have
  been deleted.

* Some libraries that we expect to be updated regularly continue to be
  provided as RPM packages.

* As a consequence, RPM packages can drop many BuildRequire
  dependencies and add xs-opam-repo as a new dependency. The %build
  and %install sections sometimes change slightly.

* Some RPM packages now download their source code from mirror
  code.citrite.net rather than github.com.