Fix params and general consistency

Hi, I used your module to create a wiki farm on a Ubuntu 12.04.4 server. I made some little improvements (IMHO) that I would ask you to pull.

As you can see from the commits, the main improvements is to strengthen the consistency of the module by substituting some magic values used throughout the code with the parameters already in place (in class mediawiki::params) for those values. Furthermore, I added some parameters default, updateed the default tarball_url with that of the latest supported legacy release and fixed minor issues.
Please contact me if you want to discuss these changes.

Ciao
Dario Sestero