sshd: Warn about missing ssh-keygen only when necessary

The sshd service is using ssh-keygen to generate missing SSH keys.
If ssh-keygen is missing, it prints the following message:

/etc/rc.d/sshd: WARNING: /usr/bin/ssh-keygen does not exist.

It makes sense when the key is not generated yet and
cannot be created because ssh-keygen is missing.

The problem is that even if the key is present on the host,
the sshd service would still warn about missing ssh-keygen
(even though it does not need it).

Reviewed by: emaste
Approved by: emaste (src)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D23911