Previously, kubeadm would use <ip>:<port> to construct a master
endpoint. This works fine for IPv4 addresses, but not for IPv6.
IPv6 requires the ip to be encased in brackets when being joined
to a port with a colon.
This patch updates kubeadm to support wrapping a v6 address with
[] to form the master endpoint url. Since this functionality is
needed in multiple areas, a dedicated util function was created.
Fixes: https://github.com/kubernetes/kubernetes/issues/48227
Allow to specify a node-name instead of relaying in `os.Hostname()`
This is useful where kubelet use the name given by the cloud-provider to
register the node.
Partially fix: kubernetes/kubeadm#64
