Support for separate Keystone paste.deploy configuration

Support for Keystone change https://review.openstack.org/24126
while keeping the backward compatiblity with mixed cfg/paste.deploy
configuration file.

Also remove insertion of s3 extension which was merged in
https://github.com/openstack/keystone/commit/b2aa620bc9d040f803c49707237f43ff1a04ef93

Change-Id: Ib7e2913ebb507f882dcd71b1142bcdb4b040ae6b
Merge "Support for separate Keystone paste.deploy configuration"