Add load-balancing based on haproxy.
Allows layer 4 and 7 balancing.
Proposed CLI
```
set load-balancing haproxy frontend fr01 backend 'bk01'
set load-balancing haproxy frontend fr01 mode 'http'
set load-balancing haproxy frontend fr01 port '8888'
set load-balancing haproxy backend bk01 mode 'http'
set load-balancing haproxy backend bk01 server srv01 address '192.168.122.11'
set load-balancing haproxy backend bk01 server srv01 port '8881'
set load-balancing haproxy backend bk01 server srv02 address '192.168.122.11'
set load-balancing haproxy backend bk01 server srv02 port '8882'
```