As interface is set to L3 mode, reset its output_node_index for l2-output

As interface is set to L3 mode, reset its output_node_index for l2-output

As an interface is changed from L2 bridging/xconenct to L3 mode, reset the
output_node_index for l2-output for the interface's sw_if_index. The reset
needs to be done in case the sw_if_index of this interface is freed and
reused for another interface type afterwards. Otherwise, the stale output
node may be used by l2-output for this sw_if_index and result in undesirable
behavior including the possibility of VPP crash.

Change-Id: Idd91d288c7643f0a1975a6787fcc330d03a7fac7
Signed-off-by: John Lo <loj@cisco.com>