Don’t install PKGNG on 9 if it already exists

There are instances where the base FreeBSD 9 install will already
contain a fully configured `pkg`. The ‘official’ EC2 AMIs follow this
pattern:

  http://www.daemonology.net/freebsd-on-ec2/
Merge pull request #15 from opscode-cookbooks/schisamo/pkgng-installed

Don’t install PKGNG on 9 if it already exists