add `--daemon` to `step ssh renew ...`

[tmcolby]

What would you like to be added

consider adding support for the --daemon argument to the step ssh renew .. command

Why this is needed

similar to the step ca certificate renew --daemon ... we need a way to automate ssh host certificate renewal.

[Duck-dave]

In the systemd service for automatic renewal, I see a command ExecCondition=/usr/bin/step certificate needs-renewal $CERT_LOCATION, would be great if we can have the same command for ssh.

[dopey]

@Duck-dave https://github.com/smallstep/cli/blob/master/command/ssh/needsRenewal.go -- we do!

I believe it's already available in the latest release on github.

Edit: Ah shoot! My mistake. It's not in v0.16.1, but it's already in master and will go out with the next release.