KVM: replace hardcoded network script path

Currently the kvm automatic network scripts accepts to be overridden by
an user supplied /etc/ganeti/kvm-vif-bridge script. We keep this
functionality but move the hardcoded path to a constant, dependent also
on SYSCONFDIR.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>