labs_vagrant: Use hiera.yaml and set realm

Tell puppet where to find the MediaWiki-Vagrant heira configuration file
when invoked via `labs_vagrant provision`. Also set a facter fact for
'realm' == 'labs' to allow hiera and puppet to switch behavior. This
will be used in a follow up change (I87e80f2) to introduce per-realm
heira configurations.

Change-Id: Ie8012e695d05eb5f6eb5b6cc1894efa64a3d4f2c