Fix for policy regression failures in R2.0 - Build 8.

1. regular expression library import was missing from
common/policy/policy_test_utils.py
   Fixes test cases:
        test_policy_RT_import_export
        test_policy_single_vn_modify_rules_of_live_flows
        test_policy_multi_vn_modify_rules_of_live_flows
        test_policy_replace_single_vn_modify_rules_of_live_flows

2. exception check for scappy stop traffic was not there in
serial_scripts/policy/test_policy_serial.py
   Fixes test cases:
        test_policy_with_implict_rule_proto_traffic

Change-Id: I7246d4ebaa61da83a9c6cca23e33708859f6018b
Merge "Fix for policy regression failures in R2.0 - Build 8."