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.
Now, defaults can be pointing to "stable" and users will always get
latest available stable build of Kubernetes via kubeadm.
There is no need anymore to hardcode version string inside kubeadm
binary.
It is also possible to use labels like "latest" or point to exact
branch: "stable-1.4"
Alexander Kanevskiy