(#5304) Use internal_name rather than real_name for maillist provider

We've switched to using list_lists --bare which shows the internal_name
rather than "real_name", which is the authoritative identifier.

Doing this also means we have less output to scrub, so the code is simpler.

Modified by Markus as-per discussion with Nigel.

Signed-off-by: Nigel Kersten <nigel@puppetlabs.com>