Add `config` attribute to hook interface

Previously, the only way to access a hook's custom configuration was to
use `@config`. Add a `config` attribute to clean this up and make the
interface more formal.

Change-Id: I8d7d4f7ceda9ded0f62650642e076053b3bf39e5
Reviewed-on: http://gerrit.causes.com/40978
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>