Remove metadata/metadetails from instance/server groups

This removes the half-implemented metadata implementation from server
groups. This first patch removes all uses of metadata/metadetails,
except what is required to handle existing API users that expect to see
an empty metadata key, and existing RPC clients expecting the same in
their InstanceGroup objects.

Change-Id: Ied2e8b058f9848e76222ca86e8f79105f08ea938
Merge "Remove metadata/metadetails from instance/server groups"