Move heat-api auth_token conf from paste.ini

Move heat-api auth_token conf from paste.ini

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

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

Change-Id: Ia0a4d912cd7380094e121ee4af733277ca4d812e
Blueprint: keystone-middleware