* Add text explaining the basics of how the ansible provisioner works.
* Refactor the basic example to demonstrate the simplest case.
* Use active voice more consistently.
* Clearly separate host signer and user key generation into separate
functions and data structures.
* Remove inaccurate comment about needing to specify both files if
either one is specified.
* Rename parameters for clarity according to their meaning to the
callee.
* Style the code with gofmt.
