Consolidate mediawiki::users::* into a single class

* Collapse mediawiki::users::{mwdeploy,l10nupdate,sudo} into a single class.
* Move l10nupdate's authorized_keys to a file
* Use native user / group rather than systemuser.

Change-Id: I44df6e7f9706ff565eae3334e4d1574001df06dc