Sync config generator from oslo-incubator

Sync from oslo-incubator with the following patches about the config
generator:

7c18261 Temporary workaround for config generator
4b3e32b Sort the output of config/generator.py by group name
806272e Improve error reporting on import failures
6cd1c33 Clean up extra modules code
3d077a3 Print the deprecated group/name for each option
aec6d94 config.generator: allow extra modules importing
ddc4f0d Fix error when no custom config file matches *.conf.sample
d4bf84d Allow generate_sample MODULEPATH to be set in env

UpgradeImpact

DocImpact, the default values in keystone_authtoken config group are
changed.

Change-Id: If7181e14db2e43e42255bb6221d0543c13c4e5a6
Merge "Sync config generator from oslo-incubator"