Fix docstring for _Namespace._get_cli_value

Change-Id: Idea5f7c57c5e35f6de74bc7f2caac0ea6db35f35
Add section titles and fix markup of docstring

Add section titles to the module docstring for cfg.py so
http://docs.openstack.org/developer/oslo.config/cfg.html is broken up
and easier to scan quickly.

Fix a couple of minor markup issues at the same time.

Change-Id: I03df6842b29e62de1b3c923218f174cf65fdefdb
Reject option names prefixed with '_'

Such names are used for _Namespace's internal purposes.

Change-Id: I08c24f05ec3ef626b253ca3682439c1a162959e5
Related-Bug: #1284969
Merge "Reject option names prefixed with '_'"
Merge "Fix docstring for _Namespace._get_cli_value"
Merge "Add section titles and fix markup of docstring"