require sections are specified in nova_config

This massive code commit actually implements something very simple.

previously, we allowed nova_config to omit a section and assumed that
section was default.

This commit updates the code to require section names for all settings.

This change is being made b/c:
- it better maps to the config on disk
- it is consistent with the other modules

Change-Id: Iae71a4c48ed0f9792566f16f0bf13e61569b46e5