100% test coverage for security groups and rules

This patch achieves full test coverage for security_group and
security_group_rules by:

- Fixing the arguments used in test_invalid_parameters_create
- Testing the __str__ and delete methods of SecurityGroupRule
- Adding a test for the ___str__ method of SecurityGroup

Change-Id: I9cfbc68761f158754aa4b339238d29cc587c91e1
Merge "100% test coverage for security groups and rules"