puppet: pin packages to the correct version

As puppet 3 packages are introduced in the wikimedia repository for
precise, and we begin the transition, we need a reliable way to switch
versions on a class of servers.

This is provided by the $puppet_version top-scope variable, that
defaults to '2.7'; by simply overriding this value at node level will
change the version of puppet we install.

The logic of pinning is as follows: if you want puppet 2.7 on precise,
use Ubuntu packages (as puppet 3 is in the wmf repositories); if you
want puppet 3 on trusty, again use Ubuntu packages (as puppet 2.7 is in
the ubuntu repositories).

Change-Id: I89ecb28b7c09c78b7ee59b88c224b68d8e0e06e2
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>