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

[tanganellilore]

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