Ansible may produce very long lines which Scanner can not handle. This
replaces the Scanner with a Reader and uses the ReadString method to
read an arbitrary large line from the ansible-playbook stdout pipe.
Ansible 2.0 deprecated ansible_ssh_user, ansible_ssh_port, ansible_ssh_host
instead use ansible_user, ansible_port, and ansible_host in the
inventory file.
Closes#3275
* 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.