We should stop special casing "ci-cross" and just use the
configuration in test-infra to dictate where we pick up
the builds from.
For 1.8,1.9,1.10 branches, we need to load the latest
version from ci/latest*.txt.
For master, 1.11 etc, we need to load from ci-cross/latest*.txt.
We need to update test-infra configs if things fail.
try these to verify:
```
gsutil cat gs://kubernetes-release-dev/ci/latest-1.9.txt
gsutil cat gs://kubernetes-release-dev/ci-cross/latest.txt
```
kubeadm upgrade apply now is able to utilize all possible combinations
of version argument, including labels (latest, stable-1.8, ci/latest-1.9)
as well as specific builds (v1.8.0-rc.1, ci/v1.9.0-alpha.1.123_01234567889)
As side effect, specifying exact build to deploy from CI area is now also
possible in kubeadm init command.
Fixes: kubernetes/kubeadm#451
As part of issue kubernetes/kubeadm#292 discussion, it
turned out that for users it is not always obvious that
version specification parameter must be in form "vX.Y.Z".
This patch allows to specify it in form "X.Y.Z" and
converts it internally to normal semantic version which
expected in the rest of the code.
