Fix and document environment variable KUBE_MASTERS (#7127)
Fix and document environment variable KUBE_MASTERS (#7127)

This variable was added as KUBE_MASTERS_MASTERS. That's probably a typo.
Remove the redundant `_MASTERS` suffix. Also, document the variable in the
help message.
Atoms/kubespray,kubernetes-incubator/kargo,Atoms/kubespray,kubernetes-sigs/kubespray,Atoms/kubespray,kubernetes-incubator/kubespray,kubernetes-sigs/kubespray,kubernetes-incubator/kubespray,kubernetes-sigs/kubespray,kubernetes-sigs/kubespray,kubernetes-incubator/kargo,Atoms/kubespray