clean up security groups tests

This patch clean up test codes of security groups in the
following items:
1. add a helper function create_new_security_group to replace the
   duplicate codes in test methods
2. remove duplicate test case test_security_group_create_delete
   in test_security_groups.py
3. use data_utils.rand_uuid() instead of str(uuid.uuid4())
4. import test module instead of attr

Change-Id: Iaf1a267134a12a664a0f306980c9cb39fe5ac1eb
Merge "clean up security groups tests"