Network: add disable_ipv6 and use it

Network: add disable_ipv6 and use it

Many interfaces used to get an ipv6 link-local address which
was usually unusable and therefore pointless.

In order to ensure consistency this is called in various
places:
* $bridge_add_interface() and $ovs_bridge_add_port() because
  it's generally a good choice for bridge ports.
* tap_create() and veth_create() because the activate the
  interfaces and we want to avoid the link local address to
  exist temporarily between bringing the interface up and
  adding it to a bridge.
* create_firewall_bridge_*() because firewall bridges aren't
  meant to have addresses either.
* activate_bridge_vlan() - if vlan_filtering is disabled we
  create vlan-bridges and neither them nor their physical
  ports should have link local addresses.