vpnmain.cgi: Add option to regenerate the host certificate

vpnmain.cgi: Add option to regenerate the host certificate

This is necessary since we now have a much shorter lifetime for the host
certificate. However, it is complicated to do this is which is why we
are copying the previous certificate and generate a new CSR. This is
then signed.

A caveat of this patch is that we do not rollover the key.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>