Add config template stanza for ckan.storage.bucket

This was lost in e0cd7ba46e260b503b4090d1a382c5bc5dcf7db8
Better config stanza and doc for ckan.storage
Merge branch 'config_doc'
Merge branch 'master' of github.com:okfn/ckan
Merge branch 'master' of github.com:okfn/ckan into 943-writing-extensions-tutorial