Format optional

cu.load(config_string, format='xml/set/text')

can now be called directly like:

cu.load(config_string)
https://github.com/Juniper/py-junos-eznc/blob/master/lib/jnpr/junos/utils/config.py#L270

even if we have have set as format it finally gets converted to text. 

https://github.com/Juniper/py-junos-eznc/blob/master/lib/jnpr/junos/utils/config.py#L274-L275
(this is existing functionality)