Add postflight for OSX packages to add puppet user, group

Previously, users installing the Puppet package for OS X would need to create
their own user and group to run `puppet apply` successfully or to set up a
puppet master. This also was out of line wiht our other puppet packages, which
do create the puppet user and group. This commit adds a postflight script to
the OS X packaging that will create a puppet user and group if they don't
already exist.

Note: this commit depends on packaging changes to install the postflight during
OS X packaging and will be inert until that change is made (the packaging change has been merged).