Use HOST_IP instead of SERVICE_HOST for heat API conf

Heat config values heat_metadata_server_url, heat_waitcondition_server_url and
heat_waitcondition_server_url currently derive their host from devstack
SERVICE_HOST. In gating this is set to 127.0.0.1, which would explain why
nova servers are not reaching heat with waitcondition signalling.

This change uses HOST_IP as the default instead of SERVICE_HOST.

Change-Id: I373b086e3a36a3484cfd34f0d1c8c168ac6d465d