Set sensible default for keystone's paste

The https://review.openstack.org/#/c/66247/ change introduced
the requirement to set config_file before keystone would start
up. Restore the old setting as default to allow a smooth
transition for users.

Change-Id: I935c5144728d04c59abaee9a515e249ef0d24477
Merge "Set sensible default for keystone's paste"