ovs-docker: Add the ability to set VLANs.

For containers containing multiple ports and spawned by
orchestrators without openflow flows configured, it can be quite
useful to distinguish their traffic with vlans. This can be useful
when containers are spawned inside VMs instead of hypervisors.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>