openssh: replace individual ssh-keygen calls with a single call

Since openssh-6.0, the ssh-keygen app has supported a -A option,
which creates any missing keys.  This frees us of having to add
new ssh-keygen invocations as new key types are introduced.  This
also frees us of having to know the default key names and locations.
So this patch replaces all the the init.d script invocations with
a single "ssh-keygen -A" call.

Note: the systemd service script *already* uses this option.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>