CC-54: Cleanup postinst / postrm scripts for chef-dk

@lamont-granquist I've opted out of reducing the code duplication between chef and chef-dk for now. I'd vote to wait and see if they will converge / diverge more and the next time we're touching here we can re-engineer it a little bit. 

CI test: http://manhattan.ci.opscode.us/job/chefdk-build/architecture=x86_64,platform=ubuntu-12.04,project=chefdk,role=builder/17/

LMK what you think...