Set $server_url to something other than undef

If $::forwarded_url is undef, set $server_url to "http://127.0.0.1".
This avoids puppet/modules/mediawiki/templates/parsoid.localsettings.js.erb
putting
  parsoidConfig.setInterwiki( 'localhost', 'undef/w/api.php' )
in /srv/parsoid/js/api/localsettings.js in labs-vagrant, which won't
work.

Fix from Erik Bernhardson.

Bug: 57701
Change-Id: I42a6bb49de5fd78ce79a2ff75b606c240c209026