clarify cluster startup/invocation in the documentation.
clarify cluster startup/invocation in the documentation.

trimming some redundancies in the docs by stating unambiguously that a single
`vagrant up` invocation is all that is needed to fire up all the needed VMs.

Signed-off-by: António Meireles <9056f7c1f560fe4e0c75ab3cde8bee0cf3d504f4@reformi.st>

duncanfinney/kubernetes-vagrant-coreos-cluster,AntonioMeireles/kubernetes-vagrant-coreos-cluster