ovpnmain.cgi: Bug 12574 - OpenVPN Internal server error when returning after generati...

ovpnmain.cgi: Bug 12574 - OpenVPN Internal server error when returning after generating root/host certificates

- Option "--secret" was deprecated in OpenVPN 2.4 and removed in OpenVPN 2.5
   It was replaced by "secret". If "--secret" is used with genkey then a user warning is
   printed and this is what gives the Internal server error.
- Patch was defined by Erik Kapfer but currently he does not have a build environment
   so I have submitted the patch on his behalf.
- Patch tested on a vm testbed running Core Update 160. Confirmed that without patch the
   error still occurs and with patch everything runs smoothly.

Fixes: Bug #12574
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by : Erik Kapfer <ummeegge@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>