Refactor tests for Interface external_ids

This change refactors the test procedure for Interface external_ids.
Functionality is added to perform "update" tests for the Interface external_ids
field.  The existing test case for Create, Read & Delete of Interface
external_ids is consolidated with the new update test procedure resulting in
one test containing 16 test cases for full CRUD coverage.

Change-Id: I92bf2532cdd2f005ddbc736b8c7de7bbb82efdea
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>