rdma: add support for promiscuous mode

rdma: add support for promiscuous mode

rdma interfaces filter packets per MAC by default to share the physical
interface between multiple users (eg. VPP and Linux).
When configured in promiscuous mode, all packets will go to this
interface, regardless of the MAC. All other interface will not receive
anymore packet while it is in promiscuous mode.
Promiscuous mode is needed (and automatically turned on) for L2 path
(l2patch, xconnect, bridge...).

Change-Id: I4c0eb4421f51d116e635e7828d00f202f4a97ded
Signed-off-by: Benoît Ganne <bganne@cisco.com>