Use $is_pe for PE determination

...rather than the custom logic that was used previously. Using a
uniform canon fact across all modules that need to differentiate between
PE and POSS makes unusual situations (such as having a missing
/opt/puppet/pe_version file) more, well, uniform.

The hobby work that motivated me to submit this has to do with
puppetizing puppet and pe and the need to have one and only one set of
criteria to pre-configure to get a skeleton PE installation recognizing
itself as PE, before passing on management work to Puppet and modules
like this one.
Ignore metadata.json file

Until this becomes the authoritative piece of metadata, it's just an
artifact.