Add Terraform support

[titilambert]

Hello,
In my company, we wrote a small k8s installer based on Terraform and Ansible.
I would like to use kubespray instead of it, so I need to add some feature to kubespray and kubespray-cli.
The first one is the Terraform support.
Are you willing to accept that kind of contribution ?

Thanks !

[titilambert]

First version:
titilambert@7e8318b

[Smana]

thanks @titilambert and sorry for the delay, that contrib would indeed be really useful!
Could you please open a pull request with the related documentation as well ?