make linode_create idempotent and integrate it into role

[skyfaller]

Last time I checked, linode_create.yml wasn't idempotent, meaning that we can't safely run the playbook on existing linodes to make sure they're set up correctly.

• [ ] clean up ~/.ssh/known_hosts in idempotent fashion
• [ ] only force newly created users to change their passwords, rather than everyone
• [ ] merge code with user-creating tasks in main.yml
• [ ] accept an array of linodes, rather than only a single linode
• [ ] support multiple OSes, including Arch Linux
• [ ] prepare for the possibility that not every server will be hosted on Linode in the future
• [ ] once idempotent, move playbook into role