addons: vlan: new attribute: vlan-bridge-binding

addons: vlan: new attribute: vlan-bridge-binding

From Mike Manning:
In the case of vlan filtering on bridges, the bridge may also have the
corresponding vlan devices as upper devices. Currently the link state
of vlan devices is transferred from the lower device. So this is up if
the bridge is in admin up state and there is at least one bridge port
that is up, regardless of the vlan that the port is a member of.

The link state of the vlan device may need to track only the state of
the subset of ports that are also members of the corresponding vlan,
rather than that of all ports.

Add a flag to specify a vlan bridge binding mode, by which the link
state is no longer automatically transferred from the lower device,
but is instead determined by the bridge ports that are members of the
vlan.
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Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>