Virtual LAN (VLAN) support for Open vSwitch and physical interfaces

If you want to attach the container to a specific VLAN, the VLAN ID can be
specified using the `[MAC]@VID` notation in the MAC address parameter.

**Note:** VLAN attachment is currently only supported for containers to be
attached to either an Open vSwitch bridge or a physical interface. Linux
bridges are currently not supported.

The following will attach container zerorpcworker to the Open vSwitch bridge
ovs0 and attach the container to VLAN ID 10.

    pipework ovsbr0 $(docker run -d zerorpcworker) dhcp @10

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>