Explicitly set the value of heartbeat_in_pthread

The ``[oslo_messaging_rabbit] heartbeat_in_pthread`` config option
is set to ``true`` for wsgi applications to allow the RabbitMQ
heartbeats to function. For non-wsgi applications it is set to ``false``
as it may otherwise break the service [1].

[1] https://docs.openstack.org/releasenotes/oslo.messaging/zed.html#upgrade-notes

Change-Id: Id89bd6158aff42d59040674308a8672c358ccb3c