ovpnmain.cgi: Manually push a different gateway for static pools

ovpnmain.cgi: Manually push a different gateway for static pools

This is because in "topology subnet", ifconfig-push is massively broken.
The client is not able to configure any routes correctly by pointing
them to the interface. Instead it is trying to use the gateway address
from the dynamic pool as gateway which cannot be reached if the client
only has an IP address from another subnet. Pushing host routes is not
supported, so we have to create a hack here and pretend that there is a
gateway in the static pool somewhere.

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