Change "is lazy loaded" detection method in db_api test

This changes the test_security_group_get_no_instances() method to use a simpler
method of determining if instances is loaded on a security_group after a get
operation.

Change-Id: Ifea6c20bcaa201167ef5679f3ddb9ae8141bbbf5
Closes-Bug: #1349147
Merge "Change "is lazy loaded" detection method in db_api test"