Fix crash with worker threads on 4K VXLAN/BD setup (VPP-907)

Fix crash with worker threads on 4K VXLAN/BD setup (VPP-907)

Cleanup mapping of interface output node for the l2-output node
when interface is configured to L2 or L3 modes. The mapping is
now always done in the main thread as part of API/CLI processing,
instead of initiate mapping in the forwarding path which can be
in the worker threads.

Change-Id: Ia789493e7d9f5c76d68edfaf34db43f3e3f53506
Signed-off-by: John Lo <loj@cisco.com>