libcloud openstack's driver supports parameter "user-data" (namely ex_userdata) for providing user data to the instance. This is the equivalent to "nova boot --user-data" parameter.
With Openstack nova command, this results in providing the content of the pointed file. You may for instance provide a shell script, a configuration file for cloud-init...
Once the VM startups, cloud-init retrieves the file from the metadata server and uses it (executes it for shell scripts for instance).
The libcloud behaviour is different: it simply passes the parameter content as is.