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
(cherry picked from commit c32986966dfd034c3d706b2e9ab2820a2c3cfc3e)
Merge "Fix security group list when not defined for an instance" into stable/icehouse