addons: vrf: add support for 'link-down yes' on VRF slaves

addons: vrf: add support for 'link-down yes' on VRF slaves

$ ifquery -a
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
	vrf mgmt
	link-down yes

auto mgmt
iface mgmt
	vrf-table auto

$ ifup -a -d
...
...
debug: mgmt: pre-up : running module vrf
info: executing /usr/lib/vrf/vrf-helper create mgmt 1001
debug: mgmt: eth0: slave configured with link-down yes
info: mgmt: netlink: ip link set dev mgmt up
...
$ ip link show eth0
2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast master mgmt state DOWN mode DEFAULT group default qlen 1000
    link/ether 08:00:27:80:e2:97 brd ff:ff:ff:ff:ff:ff

Reviewed-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>