ovpn: Generate ta.key before dh-parameter

ovpn: Generate ta.key before dh-parameter

Fixes: #11964 and #12157
If slow boards or/and boards with low entropy needs too long to generate the DH-parameter, ovpnmain.cgi can get into a
"Script timed out before returning headers" and no further OpenSSl commands will be executed after dhparam is finished.
Since the ta.key are created after the DH-parameter, it won´t be produced in that case.
To prevent this, the DH-parameter will now be generated at the end.

Signed-off-by: Erik Kapfer <ummeegge@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>