Proposal: support CNI network plugins

[WeiZhang555]

Currently runv master can't operate with CNI network plugins, I think we should add some network operation interfaces for runv to make it work with CNI. Interfaces could be like this:

Commands:
	interface	manage network interface
	route		manage route

1. runv interface

Subcommands:
	add	add an interface into a container
	rm	remove an interface from a container
	update	update configuration of an interface
	ls	list network interfaces in an container
	inspect show configuration of a specific network interface from a container

1.1. add

usage: add container
flags:
	--tapname	use existing tap device in host, tap will be created for you if empty
	--name		set interface name in container
	--ip		set ip address with a mask, format: 192.168.0.2/24
	--mac		set mac address 
	--mtu		set mtu

1.2. rm

Usage: rm container name - rename interface "name" from container "container"
flags:

1.3 update

Usage: update container interface - update configuration of interface "interface" in a container
flags:
	--name		set interface name in container
	--add-ip	add one more ip address with a mask, format: 192.168.0.2/24
	--rm-ip		remove one ip address from a container, format: 192.168.0.2/24
	--mac		set mac address 
	--mtu		set mtu

1.4 ls

Usage: ls container - list all interfaces in a container

2. runv route

Subcommand:
	add	add route into a container
	rm	remove route from container
        ls      list all route rules in a container

2.1 add

Usage: add container route - add route rule into a container
flags:
	--dest		the destination prefix of the route
	--dev		the output device name
	--src		the source address to prefer when sending to the destinations covered by the route prefix.
	--gateway	the address of the nexthop router

2.2 rm

Usage: remove container route - remove route rule from container, "route" is destination prefix of the route

2.3 ls

Usage: ls container - list all route rules in a container