Emit mediawiki-ready Upstart event

Events emitted by upstart-file-bridge and mountall don't seem to be reliable,
so do a busy-loop instead, as suggested by:
  <http://upstart.ubuntu.com/cookbook/#run-a-job-when-a-file-or-directory-is-created-deleted>.
Use the event as a way of starting Upstart services that are dependent on
MediaWiki at the right time.

Change-Id: I09f0e44fb23e6ed6534124b18c6126488401fcf7