tests/coreos-install: update version only test on cl machines
tests/coreos-install: update version only test on cl machines

Updates the logic of the Version Only test to use the `GetDefaults`
method to determine the Channel and selecting a version based off
of that. The previous version of this test would fail when ran on
a Container Linux machine that was not on the stable branch.

coreos/init,coreos/init,coreos/init