This moves all existing `image/services/before_script/variables`
into `default:`. This allows us to easily add a default and
top-level entries. `default`: is keep backward compatible: to
be considered to be job if `default:script:` is specified. This
behavior should be removed.
All existing `image/services/before_script/variables` are properly
handled in root context.
Introduce `default:` configuration entry setting that makes it possible
to configure a default value of an entry, what overrides class-level
`def self.default` value.
