Glance: Add configuration hierarchy

Currently glance configuration parameters are not hierarchical. It
makes sense to add a hierarchy for API/Registry/Scrubber etc, such as
glance.api.<param>, glance.registry.<param>, glance.scrubber.<param>.

This patch makes a start at that. Once this merges the json/yaml
files will be updated to use the new style; the old style params
can then be retired.

Change-Id: Ie151978024042275f07f2d8e42aca113801af38e
Merge "Glance: Add configuration hierarchy"