This updates the HPA controller to use the polymorphic scale client from
client-go.  This should enable HPAs to work with arbitrary scalable
resources, instead of just those in the extensions API group (meaning we
can deprecate the copy of ReplicationController in extensions/v1beta1).
It also means that the HPA controller now pays attention to the
APIVersion field in `scaleTargetRef` (more specifically, the group part
of it).

Note that currently, discovery information on which resources are
available where is only fetched once (the first time that it's
requested).  In the future, we may want a refreshing discovery REST
mapper.