Changes SSH key behavior such that there is a single persisted key for all
machines across all providers. If there is no key that is located at
`.local/share/containers/podman/machine/` then it is created. The keys are
not deleted when the last machine on the host is removed.
The main motivation for this change is it leads to fewer files created on the
host as a result of vm configuration. Having `n` machines on your system doesn't
result in `2n` machine-related files in `.ssh` on your system anymore.
As a result of ssh keys being persisted by default, the `--save-keys` flag
on `podman machine rm` will no longer be supported.
Signed-off-by: Jake Correnti <jakecorrenti+github@proton.me>
