VPP-110: vxlan encap node should never touch a deleted tunnel

VPP-110: vxlan encap node should never touch a deleted tunnel

Remove usage of dummy encap string for output from BD to a tunnel which
has been deleted. Instead, use a node l2-outpt-del-tunnel so that if
there are stale entries in the L2FIB for any deleted tunnel sw_if_index,
l2-output will send packets using this entry to the l2-output-tunnel-del
node which just setup the proper drop reason before sending packets
to the error-drop node to drop the packet.

Change-Id: I590982ee25e924ab74e2855c55c58baf29a9fad4
Signed-off-by: John Lo <loj@cisco.com>