ovpnmain.cgi: Fixes bug#13404 - prevents certs being saved if common name is already...

ovpnmain.cgi: Fixes bug#13404 - prevents certs being saved if common name is already used

- This was fixed by moving the code for checking if the common name is already used, to
   the same location as the code for checking if the connection name is already used.
- Tested out on vm testbed and confirmed that the certificates are not created and the
   index.txt not updated if the common name is flagged as already being used. If the
   entry is changed to use a new CN and Save pressed then the certs are saved and the
   index.txt updated. If Cancel is pressed then no certs are saved and index.txt is not
   updated.

Fixes: Bug#13404
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>