(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.  Track the relevant
   SHAs with the modified code so that dependent versions are tracked
   in the source tree, rather than through transient Jenkins jobs.
 - To allow the jobs to build a particular architecture, introduce a
   new environment variable called WIN32_RUBY_ARCH that can be used to
   select either 'x86' or 'x64'.  If an arch is not specified, fall
   back to 'x86', but if the appropriate :rev is not specified, kill
   the setup step.
(maint) Use an absolute path so we're not sensitive to pwd

Previously, the location of the windows versions.yaml file depended on
your current working directory, and assumed it was the root of the
project. But acceptance tests are typically executed by cd'ing into
acceptance, and then executing rake tasks from there.

This commit ensures we reference the versions.yaml relative to the
currently executing 000_EnvSetup.rb setup step.
Merge branch 'pr/2801'

* pr/2801:
  (maint) Use an absolute path so we're not sensitive to pwd
  (PUP-2828) puppet-win32-ruby acceptance ver config

Closes GH-2801