netfilter: add ip_vs kernel modules to enable load balancer

netfilter: add ip_vs kernel modules to enable load balancer

IPVS (IP Virtual Server) implements transport-layer load balancing inside
the Linux kernel, so called Layer-4 switching. IPVS running on a host acts
as a load balancer at the front of a cluster of real servers, it can direct
requests for TCP/UDP based services to the real servers, and makes services
of the real servers to appear as a virtual service on a single IP address.

This change adds kmod-nf-ipvs and kmod-nf-conntrack-ftp kernel modules
options to LEDE kernel netfilter

Signed-off-by: Mauro Mozzarelli <mauro@ezplanet.org>
[Jo-Philipp Wich: replace kmod-nf-conntrack-ftp with kmod-nf-nathelper
                  dependency, rewrap commit message, adjust subject]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>