Decrease number of simultanous fastd connections by one
See https://github.com/freifunk-gluon/gluon/pull/494 for
further discussions. The reason for two simultanous connections
was fault tolerancy. But this benefit is payed by more vpn
traffic. Our network has grown up to more than 800 online nodes.
Thus, we set the number of fastd connections to one which hopefully
lowers the amount of bandwith, a node uses.
