Revert "Support lookup of value using "group.key""

This reverts commit 525ac47.

There are already two ways to reference an option in a group:

  CONF.group.key
  CONF[group].key

Adding a third variant doesn't seem ideal. Also, for the specific case
of LazyPluggable in Nova, we can easily just pass an optional config
group name to the constructor.

Change-Id: I1a29a18d90e8af3ce2563bd1b3eeb64422140016