add argument for postgresql port in datastore_setup.py

reference issue 1738.

I already had a few versions of postgres running - oldest on port 5432, middle on port 5433, and newest (where I wanted to put ckan) on port 5434.  There was no way to specify the port number to the datastore setup script, so I added a command line arg.

I'm not sure how to make tests for this, suggestions welcome.