Introduce SCM module in Puppet

[matze]

(As discussed with @trev and @fhd on IRC:) We have a few dozen exec {... } based invocations of hg(1) in our infrastructure repository, and almost as many different versions when looking at the details. This should become abstracted in order to not repeat ourselves and to remove unnecessary possible failure sources.

Goal

Introduce a Puppet module that abstracts the invocations and replace the aforementioned exec {... } directives accordingly.

Notes

There's probably plenty of similar modules out there already, so we may not need to write a new one from scratch. Still, since it's a relatively trivial domain, we may not want to introduce another explicit dependency for this. Especially since we may want to also integrate the ensure_dependency.py script based on this module, and thus may need to apply more patches to any existing module than an individual solution would require.

[matze]

Status: @AAlvz, @poz2k4444 and I have actually already started looking into a set of modules for Puppet yesterday; we've just missed the ticket today and realized there's none yet.

[matze]

With our recent plans to use bookmarks as feature branches, i.e. master, we have to address most of the code snippets in question for this ticket here anyway. Hence I do consider the implementation more urgent now, and also include the requirement for post-update trigger support (i.e. python ensure_dependencies.py).

Ideally, the new Puppet module should not only cover Mercurial, but also allow for using the same mechanisms with Git - in order to allow for testing new code without the requirement to clone and mirror it on hg.adblockplus.org or any other HG entities first.

[matze]

This one is obsolete.