Parametrize MediaWiki install path

I don't expect the value to be commonly overridden, but having a reference to
the value allows for writing more expressive Puppet code that interacts with
the MediaWiki setup, as the next commit will show.

Change-Id: I0892e18d585660079998de3bc8e0570b82d4fbae
Add custom resource type for MediaWiki extensions

Documented extensively in the puppetdoc block which precedes the resource
definition itself, in 'modules/mediawiki/manifests/extension.pp'.

Change-Id: I9295852940d8206d20e8f23fce20db938ba5d574