datastore: Rationalise set-permissions command

This commit reduces the number of distinct ways of setting db permissions from three to one, while simultaneously making it possible to set permissions on remote database servers without having to manually edit SQL.

The three mechanisms of doing this before were:

1) Run a paster command "set-permissions", which shelled out to
2) A script in `ckanext/datastore/bin`, which shelled out to `sudo -u postgres psql ...` and piped in a prefilled version of
3) An SQL template script

The options available for passing through to `psql` did not extend to setting remote host names, which meant that the only option for configuring permissions on remote database servers was:

- copy the SQL template script to the remote server
- manually fill in the template parameters (despite the fact they're already in the config file)
- execute the SQL

This commit replaces these with just one mechanism setting permissions, namely execute `paster datastore set-permissions` and pipe the result into an appropriate psql shell:

    paster datastore set-permissions | sudo -u postgres psql

Or maybe:

    paster datastore set-permissions | ssh postgres@dbserver psql

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*If you're looking for the stuff that allows you to execute the permissions script directly, I removed it. See  e3ae168.*