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

The git server and fork used to construct git urls for installation
projects via beaker was previously hard set to be github.com and
puppetlabs.  While we continue to default to these values, this commit
allows them to be set globally or per installed project via environment
variables so that we can take advantage of internal and local git
mirrors and daemons.
Merge pull request #2428 from jpartlow/maint/stable/acceptance-gitmirror

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