Wrap config module and require manual setup (bug 1143998)

This moves keystone.config to keystone.common.config, which requires
.configure() to be called manually in order for options to be
registered.

keystone.config preserves the existing behavior of automatically
registering options when imported.

keystone.middleware.auth_token and it's dependencies within keystone no
longer cause config options to be automatically registered.

This is an alternative to https://review.openstack.org/#/c/24251/

Change-Id: If9eb5799bf77595ecb71f2000f8b6d1610ea9700
Merge "Wrap config module and require manual setup (bug 1143998)"
Enable emulation for domains

Fixes bug #1157727

Change-Id: I3760469b8000cfc5fb461bb9ede5d0e140413dd7