Security Group w/o enough parameters should fail more gracefully

[Todd Willey]

 _revoke_rule_args_to_dict in nova/api/ec2/cloud.py can return None if the cidr and port are empty, but authorize_security_group_ingress always assumes a dict return value. We need to check for a None return value and raise an API error with a descriptive message. revoke_security_group_ingress has this check in place already.