openssh: Move key generation to the start function of init script.

Since there's not much point in generating missing host keys when the
init script is called with "stop", the call to ssh-keygen should not
be done inconditionally, but in the start function instead.

Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>