Sync config generator from oslo-incubator

Sync from oslo-incubator with the following patches about the config
generator so that adding new config group won't result the huge change
in the config generator's output.

4b3e32b Sort the output of config/generator.py by group name

Change-Id: Ibe0687711e75366a6d11442dd6b2237b1bb91e11
Merge "Sync config generator from oslo-incubator"