* Add a CoreOS test class TestCoreosKubernetesAPIs
* Add a CoreOS test environment in tox.ini
* Create a base class BaseK8sTest and move OS-agnostic k8s testing
code to that class.
* Increase the disk size from 8G to 10G for m1.magnum and s1.magnum,
since CoreOS image requires more disk space to boot.
* Set os-distro property for CoreOS image.
Partial-Bug: #1546101
Change-Id: Ie56a9442ecebe05f39c7669bc950f5a6ca11df33