Clean up system::role

* Add documentation
* Make description parameter optional, to avoid things like:

    system::role { 'misc::fundraising::mail': description => 'fundraising mail server' }
    system::role { "misc::management::ipmi": description => "IPMI Management" }
    system::role { "misc::noc-wikimedia": description => "noc.wikimedia.org" }

* Remove dead code

I'm still not sure this deserves to be its own module, but at least it's
a bit better now.

Change-Id: Ic36f85461f6eff5ef521fef4ef4ab27e91de4a4d