(PUP-2828) puppet-win32-ruby acceptance ver config

 - Previously, Jenkins acceptance jobs on Windows would clone Windows
   source for puppet-win32-ruby, the vendored Ruby that we ship as part
   of Windows MSI builds.  This comes from an internal git mirror.
 - At present, a simple checkout is performed against whatever is the
   'default' branch (typically the 1.9.3 branch).  Unfortunately, this
   will not work for any acceptance jobs that intend to use 2.0, and it
   currently doesn't work for jobs that need new versions of gems.
 - Rather than take a clone the repo and hope it works approach, make
   use of Beaker's 'rev' option when calling install_from_git.
   https://github.com/puppetlabs/beaker/blob/beaker1.11.2/lib/beaker/dsl/install_utils.rb#L106
 - Using :rev allows us to pin the repo version by using an environment
   variable that can be setup in the Jenkins job.  This variable is
   called WIN32_RUBY_SHA, but realistically can use a git commit-ish as
   specified in the git documentation for checkout -f
   https://www.kernel.org/pub/software/scm/git/docs/git-checkout.html
   It will accept a branch, tag, SHA, etc
 - The default :rev is set to '' so that prior behavior is preserved
Oh right on, so moving forward on master but pinning on stable. Makes sense