Test to update neutron security group

Adding update_security_group functionality to existing method
"test_create_show_delete_security_group" in test_security_groups.py.
Verifying if name and description of security group are updated.
Also using updater method from network_client_base.py for
'update_security_group' function  call.

Change-Id: I4a8b4e9d7cecf2c760b4c26d61a97cfafac25461
Merge "Test to update neutron security group"