get specific about where psql is,

 so this can easily work on a single puppet apply run.

Before this change I got the error:
"Parameter unless failed: 'psql -l |grep template_postgis' is not qualified and no path was specified. Please qualify the command or s
pecify a path."

ref: http://serverfault.com/questions/345201/how-to-set-path-when-applying-single-puppet-module