Update documentation on overriding config files

The main motivation here is to document a mechanism which can be
used to configure Nova cells on a per-cell basis without introducing
a myriad of additional locations to put config files. The
following changes are made:

- Remove the note about only ini files being supported because
  merge_yaml is now used
- Expand on supported config file locations
- Add a section on using conditionals in the config file

Partially Implements: blueprint support-nova-cells
Change-Id: I92599e501506fdacaf3adb94cc6fffcf6fea2af3