It turns out that the portmap plugin is not idempotent if its
executed in parallel.
The errors are caused due to a race of different instantiations
deleting the chains.
This patch does that the portmap plugin doesn't fail if the
errors are because the chain doesn't exist on teardown.
Signed-off-by: Antonio Ojea <antonio.ojea.garcia@gmail.com>
3603738c6a