Fix add policy missing rule issue

In firewalls/workflows.py retrieve 'rule' instead of 'firewall_rules'
from AddPolciy post message. In test_add_policy_post, we need to
ensure 'firewall_policy_id' field is not set before added to policy.

Change-Id: I5f9ff81f42f4f8c7ce5c638c9074cc2dec4e1434
Closes-Bug: #1231235
Merge "Fix add policy missing rule issue"