Updating git::clone so that gerrit urls can be assumed by default

e.g. git::clone { 'analytics/wikimetrics': directory => '/srv/wikimetrics' }

This will clone analytics/wikimetrics from gerrit.

This is also for compatibility with the mediawiki-vagrant git::clone define,
so that submodules shared between mediawiki-vagrant and operations/puppet
and that use git::clone will work in both.

Change-Id: Id76ff8bd9896d188d0a2c65d19e9d456be01d22f