mediawiki: manage single configs via apache::site (one server))

With this change, we move one more step in the direction of managing all
the configuration of our appservers via puppet:

- sites are now local to puppet, not in a submodule
- being managed by apache::site just means we can deploy changes and the
  graceful restart will be managed by puppet itself without risking
  harm.
- The redirects.conf file still needs to be generated within
  apache-config and submitted to puppet; this will change soon

For sake of testing, the change is limited to be applied to the staging
host.

Change-Id: I291f1c21c7d000eb78b37d7ab66ece300fe07eb5
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>