Add ability to set per netns / interface sysctls

[cooperlees]

This could be handy to test Linux sysctls in different netns's.

Could add a sysctl dict to each interface:

{
    "namespaces": {
        "left": {
            "id": 1,
            "interfaces": {
                "left0": {
                    "sysctls": {
                        "net.ipv6.conf.left0.mc_forwarding": 0
                    },
                },
                    "type": "veth"
                },
}

Or / and a per namespace sysclt stanza ... so it gets applied to ALL interfaces ...