use default args if no args are passed

I am using this and importing it in my own class.  The only way that it
should be empty (and this class being used) is if you are manually
loading extensions, so this shouldn't affect anyone that just uses
novaclient.v1_1.servers directly, unless they manually load these
portions.  You can also disable them by putting public: False and
service_net: False into the kwargs passed to the servers.create()
function