Better fix of #212

The manifest-from-url feature of kubelet will now accept either single or arrays of manifests.

I'm adding a bit to integration test that makes sure our container vm guestbook example causes a container to be created. Right now I'm just pulling it from https://raw.githubusercontent.com/GoogleCloudPlatform/container-vm-guestbook-redis-python/master/manifest.yaml every time. Do you think I should cache it locally? I kind of like pulling it every time because that ensures that our documentation and reality will stay aligned, but it also makes the test depend on github...
We already started using github.com/stretchr/testify/mock in kublet_test.go. We should standardize now, before we get too large of a mix of testing methods. But not in this PR.