fix scaling in kubeadm etcd mode (#6822)

'ansible.vars.hostvars.HostVarsVars object' has no attribute 'kubeadm_upload_cert'

kubeadm_upload_cert will never be found as a hostvar for the first
master since the task is executed for a worker.

Fix by executing the upload task for the first master and register
the needed key. After that, workers can read hostvars for the master

Var kubeadm_etcd_refresh_cert_key removed since it no longer has
any use.