Add cfgfilter.ConfigFilter

Add a new class designed to wrap cfg.ConfigOpts, with the following use cases
in mind:

1. Help enforce that a given module does not access options registered by
   another module, without first declaring those cross-module dependencies
   using import_opt().

2. Prevent private configuration opts from being visible to modules other than
   the one which registered it.

blueprint: oslo-config-cfgfilter
Change-Id: I477f7027806d7630ce91159fb274f41c0c203770
Merge "Add cfgfilter.ConfigFilter"