mediawiki: manage the apache config via puppet

This is the first in a series of changes that will allow to manage
the apache config for mediawiki using a more standard layout. For now we
just deploy apache-config via puppet using a directory resource, and
importing apache-config as plain files with a submodule.

We introduce tags so that it should be possible to release apache-config
by simply:

1) merging the change in apache-config
2) update the submodule
3) run on the saltmaster:
   salt -G 'cluster:appserver*' cmd.run 'puppet agent -t --tags mw-apache-config'

which should be simple enough. Also, puppet will ensure and enforce this
config on servers that were down at the moment of the deploy.

In this change, we leave the feature turned off by default, apart from
the staging server (mw1017)

Change-Id: I43126e44e96b8ae88eb5ad1958b1ce6fe5ade9f5
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>