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

To avoid huge changes in the output when adding new option groups.

Sync from oslo commit 4b3e32b980cf1d028da9325b21183ff635fbe043.

Closes-Bug: #1258363

Change-Id: I9173a38d4d548ce641e19ada30f86148c1f0c53f
Merge "Sort the output of config/generator.py by group name"