mark perlbrew user/group as system; allow system to assign ids

On many systems we will want to allow the system to choose a numeric
user/group id for us, so as to avoid any conflicts.  And I suspect we always
want to mark this account as a system account, as that has special
implications.
update tests for new user/group options
Merge pull request #15 from perl-puppet/topic/allow-system-ids

mark perlbrew user/group as system; allow system to assign ids