Fix a few bugs in flow modifications

- Distinguish between dangling references to contracts and
deleted contracts in PolicyManager
- Set the return value correctly in FlowExecutor
- Don't add policy-table flows if there are no classifiers

Change-Id: I5506885b36fb5dd2515aa4b6824cd88cea54ca98
Signed-off-by: Amit Bose <bose@noironetworks.com>