(#6770) Don't pollute valid face list when #face? is called.

We had two conflicting uses of the list of available faces: in the #face?
method we were perfectly happy to create a top level key on any request, but
didn't populate the version set.

Meanwhile, in #faces we treated the set of top level keys as the absolute and
correct list of all *valid* faces, leading to pain and suffering when people
queried for an invalid face, but then expected to enumerate only valid faces.

Paired-With: Matt Robinson <matt@puppetlabs.com>