Some refactoring of the cfg cache
Some refactoring of the cfg cache

A fairly misc bunch of changes:

 - init cache before registering config-file and just let
   register_cli_opt() clear the empty cache

 - use @__clear_cache on set_default() and set_override() since
   these are just used by the unit tests and doing so allows us
   to kill _remove_from_cache()

 - use @__clear_cache on reset() too

 - remove recursion from _get() and the substitute param

 - just use (group_name, opt_name) as the cache key

Change-Id: I66934e748eca9ec03e44d7f80a7e10d96a77d8eb

tianshan/oslo.config,metacloud/oslo.config,JioCloud/oslo.config,varunarya10/oslo.config,citrix-openstack-build/oslo.config,magic0704/oslo.config,citrix-openstack/build-oslo.config,shad7/oslo.config