remove default owner/user and group values

The use of $::id to set the default user/owner and group has caused
multiple bugs in the past, is incorrectly used to infer the egid,
introduces a dependency on the `id` fact, and provides no functionally
that can't be accomplished by passing `undef` or not setting the
respective params on the file & exec types.

A possible alternative would be to introduce a dep on the $::gid fact
but that would mean the entire module would depend on a version of
facter than hasn't shipped yet (unworkable) or to add a gid/egid fact
into this module (ugly).
Merge pull request #85 from jhoblitt/remove_default_ownership

remove default owner/user and group values