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
Merge "Fix security group list when not defined for an instance"