Recently, the default overcloud username value has changed in the
downstream jobs
After that, some tests using the podman API failed because they
configured the podman sockets service using a hardcoded value that
doesn't apply anymore
This patch removes the harcoded value from the podman code and also from
the cloud_disruptions module
The proper overcloud ssh username needs to be configured in the
tobiko.conf file (its default configurable value is still heat-admin)
Change-Id: I38af9de21caf4eb2a8b222bb8f849da0cea5a323