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
Remove service _user, _password, _tenant from context

These are not currently used anywhere in heat-engine, and I'm
not comfortable with the service password going over the (controller)
network.

If heat-engine needs access to these in the future then they can always
be configured in heat-engine.conf, or the operation can be performed
in the api and the results put into the context.

Change-Id: I81aab6b5a2c9aa7d6412531f70108857cbfa637b