Add support for ssh access to gerrit

Add a new 'forward_agent' setting and ssh known host data for
gerrit.wikimedia.org. This can be used in conjunction with the `vagrant
hiera` command or manual changes to puppet/hieradata/local.yaml to
configure cloning from gerrit.wikimedia.org via ssh:

  $ vagrant config forward_agent yes
  $ vagrant hiera git::urlformat ssh://USER@gerrit.wikimedia.org:29418/%s.git
  $ vagrant up

Change-Id: I745de79f02411c11c7cf232b0f9bccc2c82415c4