Fix security group list when not defined for an instance

If an instance has an empty security group list an exception
is thrown due to iteration attempt of None.

To fix this, this patch modifies get_instance_security_group
to return an empty list instead of None.

Change-Id: I7efb3c3a58d7e20501d126c6d194479ca1d14166
Closes-Bug: #1291489
Fix wrong method name for test_hacking

There were two methods with same name, let's fix it.

Change-Id: I114ff187e1128dd042ee836ac7b24fb5bcb9c265
Merge "Fix wrong method name for test_hacking"
Merge "Fix security group list when not defined for an instance"