Allow early overriding of assets_* parameters

Because the `load:defaults` task didn't check whether `assets_roles` or `assets_prefix` had already been set, the default settings could only be overridden from `deploy.rb` or `a_stage.rb`, but not from `Capfile`. However, styles differ and keeping the relevant settings close to the appropriate `require` is possible with this change, while it has no downsides afaik.