(Maint) Allow acceptance tests to be run on preserved windows hosts

Previously, running acceptance tests on a preserved windows host would
fail, because the puppet-win32-ruby directory already existed from the
previous run, and git clone will report a fatal error.

This commit uses beaker's method for cloning a repo. The method handles
things like only cloning if necessary, resetting remotes, and cleaning
existing workspaces.

Cherry-pick of 326b7ac106fe4630ff59ec1da716207c0d67a065 from puppet