Move auth_token configurations to quantum.conf

Move auth_token configurations to quantum.conf

keystone auth_token middleware now allows quantum to have auth_token
configuration in quantum.conf. This commit moves the example of
auth_token configuration from api-paste.ini to quantum.conf.
This simplifies user configuations and users is no longer required
to edit api-paste.ini.

This change does not break backward compatibility. auth_token first
tries the configurations in /etc/quantum/api-paste.ini and then the
above configurations. Thus a user who already use api-paste.ini
does not need to change it.

DocImpact

Change-Id: I5a4c48b14428e29ea2a331880e1de0afd69c97b5