tower-cli node create not accept "extra_data" in the correct format

Hi team,

during the usage of command `tower-cli node create ... --extra-data '{key: value}'` is posted in a wrong format.
extra-data should be a json format for a correct configure inside ansible tower.

For resolve this you can change the `extra_data` field inside node.py file.

I will create a pull request that fix it.
Thanks