Add load-balancing based on haproxy.
Allows layer 4 and 7 balancing.
Proposed CLI
```
set load-balancing app server fr01 backend 'bk01'
set load-balancing app server fr01 mode 'http'
set load-balancing app server fr01 port '8888'
set load-balancing app backend bk01 balance-mode 'least-connection'
set load-balancing app backend bk01 mode 'http'
set load-balancing app backend bk01 server srv01 address '192.168.122.11'
set load-balancing app backend bk01 server srv01 port '8881'
set load-balancing app backend bk01 server srv02 address '192.168.122.11'
set load-balancing app backend bk01 server srv02 port '8882'
```
config haproxy
```
vyos@r14# cat /run/haproxy/haproxy.cfg
# Generated by /usr/libexec/vyos/conf_mode/load-balancing-haproxy.py
defaults
log global
mode http
option httplog
option dontlognull
timeout connect 5s
timeout client 50s
timeout server 50s
# Frontend
frontend fr01
bind :8888
mode http
default_backend bk01
# Backend
backend bk01
mode http
server srv01 192.168.122.11:8881
server srv02 192.168.122.11:8882
```