Fix AttachInterfacesTest error

This commit fixes some issues in AttachInterfacesTest.
* There is a problem to get the return value, TypeError is
  occurring.
* Assign the JSON or XML to _interface, but it is not specified
  in the method.
* Add a process to wait until the "ACTIVE" status.
  Because the status is "DOWN" when a interface is created.

Fixes bug 1164894

Change-Id: I71ec9e46eb15e70f870b7a833d5433fa351c42eb
Merge "Fix AttachInterfacesTest error"