SUMMARY: Fabric checked the host key against the system host key store (~/.ssh/known_hosts), which leads it to error out often in a workflow that reuses the same address for many VMs (which is common eg. on openstack). We changed the default setting to not check the host key against the system host key store. (5019 is a duplicate of this)
This is the standard SSH man-in-the-middle detection triggered by a detected change in the host key. On an OpenStack deployment, particularly if dynamic scaling is in use, the creation and deletion of nodes which re-use IP addresses from the address pool is expected.
There doesn’t seem to be a config option to override this detection. It would be nice if we could specify the expected host key, and pass it around as a runtime property, and remove host keys upon deletion of the VM.