Google Container Engine's cluster API returned instance group manager
URLs when it meant to return instance group URLs. See #4336 for details
about the bug.
While this is undeniably an upstream problem, this PR:
* detects the error, meaning it will work as expected when the API is
fixed.
* corrects the error by requesting the instance group manager, then
retrieving its instance group URL, and using that instead.
* adds a test that exercises the error and the solution, to ensure it is
functioning properly.
Some google resources required network be refernced by resource URL (aka self_link), while others required network name.
This change allows either to be supplied.
DRY it out, and add a fix for #5552.
