T959: XML/Python rewrite of "protocol igmp-proxy" and op-mode commands

Examples:
=========

CFG commands:

  vyos@vyos# set protocols igmp-proxy disable-quickleave
  vyos@vyos# set protocols igmp-proxy interface eth0 alt-subnet '172.16.35.0/24'
  vyos@vyos# set protocols igmp-proxy interface eth0 alt-subnet '172.31.0.0/24'
  vyos@vyos# set protocols igmp-proxy interface eth0 role 'upstream'
  vyos@vyos# set protocols igmp-proxy interface eth1 role 'downstream'

  vyos@vyos# show protocols
   igmp-proxy {
       disable-quickleave
       interface eth0 {
           alt-subnet 172.16.35.0/24
           alt-subnet 172.31.0.0/24
           role upstream
       }
       interface eth1 {
           role downstream
       }
   }

OP mode commands:
-----------------

  vyos@vyos:~$ show ip multicast interface
   Interface  BytesIn      PktsIn       BytesOut     PktsOut      Local
   eth0       0.0b         0            0.0b         0            xxx.xxx.xxx.65
   eth1       0.0b         0            0.0b         0            xxx.xxx.xx.201

  vyos@vyos:~$ show ip multicast mfc
  Group           Origin          Pkts         Bytes        Wrong   In      Out
  xxx.x.xx.1      xxx.xx.0.1      10           9.81KB       0       eth0    eth1
  xxx.x.xx.2      xxx.xx.0.1                                        --

0 files changed, 0 insertions, 0 deletions