Ensure a port is deleted before re-creating

This patch ensures a port is deleted before re-creating it in a
particular code path of the DatapathPortEntangler. Additionally, it
ensures that the TokenBucket hierarchy is resilient to duplicate
buckets.

Ref MNA-25
Signed-off-by: Duarte Nunes <duarte@midokura.com>

Change-Id: I6a904e24c54b337fb0916d07f07831052f7d2666