Improve security groups management.

- Add create_security_group_rule method to base class
  to make easier creating rules for IPv6 test cases.
- Add delete_security_group method.
- Make sure segurity groups are deleted using the
  client that has been used to create them.
- Improve security group client handling.

Change-Id: I50858d5198d8a70a2bb9fb278786c433d7cb12ca
3 files changed