datastore: Simplify set-permissions even further

This commit removes the ability for CKAN to execute the permissions
script for you. Why?

1. It's unnecessary. You can almost always pipe the script into a psql
   shell, regardless of the location of your database server. If you
   can't, then the only way that CKAN is going to be able to help is by
   connecting over the network. See point 2.
2. It encourages a setup in which you can connect to your database as a
   superuser over the network. This is a Bad Idea.
3. It adds substantial complication to the set-permissions command.
datastore: Explicitly connect to datastore DB in set-permissions

See comment in set_permissions.sql