(maint) Use git mirrors for puppet/hiera/facter installation

So, this will work, if we set GIT_SERVER to github.delivery.puppetlabs.net on our jobs, but it is neither fish nor fowl.  'github.com' is the default, but the config/git/options.rb had the delivery mirror hard set.  And the setup/git/pre-suite/000_EnvSetup.rb step has a hardset github path to the puppet-win32-ruby.  My current reasoning is that people outside of the vpn should be able to run source acceptance (package acceptance is right out, obviously), and so github.com should be the default.  The exception are the hiera/facter install options which may be overridden with a local_options.rb in your acceptance/ setup (or maybe that has to be done from the commandline, I'm not positive).

But it would be better to have a clear set of parameters which could be set for both the puppet installation git path (which is likely to be separate) and for supporting libs like facter, hiera, puppet-win32-ruby.

I thought I would get some opinions before proceeding further.  1) which default, github.com or our internal mirror (I vote github.com), and 2) how configurable?