Remove hardcoded postgres-user references

This is re-submit of #493, and this time I tested the way I should have.   

http://wilson.ci.opscode.us/job/private-chef-test/506/

The only difference between this and #493 is at https://github.com/opscode/opscode-omnibus/pull/573/files#diff-f14078d055d56551488acc3e3e1a8c21R225

@mmzyk