cli: Make ssh access optional

[cfergeau]

At the moment, podman machine has several expectations about what the VM image will be doing/providing:

1. ssh is running and private key/username to connect over ssh are known

2. when booting is done, a "ready" notification is sent from the guest to the host

3. ignition is used to configure the VM image. podman-machine uses this to inject ssh keys for 1) and systemd units for 2)

4. is a particularly strong assumption as the ssh connection details are passed to gvproxy on the command line to proxy a unix socket over an ssh tunnel. When these args are on the command line, if gvproxy did not manage to establish a ssh connection in a few minutes, it will exit. This means the VM will lose all its networking.

We need to make all of these optional