A common failure early on when using Kayobe is during discovery of SSH
known hosts. This happens if a host does not have an IP address
configured on the admin (SSH) network. The failure looks like this:

PLAY [Ensure known hosts are configured]
**********************************************************************
TASK [ssh-known-host : Scan for SSH keys]
**********************************************************************
failed: [compute0 -> localhost] (item=) => {"ansible_loop_var": "item",
    "changed": false, "cmd": ["ssh-keyscan"], "delta": "0:00:00.013855",
    "end": "2020-04-17 10:51:01.857855", "item": "", "msg": "non-zero
        return code", "rc": 1, "start": "2020-04-17 10:51:01.844000",
    "stderr": "usage: ssh-keyscan [-46cDHv] [-f file] [-p port] [-T
        timeout] [-t type]\n\t\t   [host | addrlist namelist]",
    "stderr_lines": ["u sage: ssh-keyscan [-46cDHv] [-f file] [-p port]
        [-T timeout] [-t type]", "\t\t   [host | addrlist namelist]"],
        "stdout": "", "stdout_lines": []}

This happens when ansible_host is an empty string, typically because the
host has no IP address defined in for the admin network in
network-allocation.yml. This is very confusing for a new user. We should
provide a more informative message.

It's not exactly clear how a user gets to this point, since the
ip-allocation.yml playbook runs before ssh-known-host.yml, which should
populate network-allocation.yml.

This change detects this failure mode and provides a message with
information about how to resolve it.

Change-Id: I564b6e4509a30dec7c49a23bb2f75d490be775ed
Story: 2007566
Task: 39456