[Feature] Etcd cluster scale-up

[aaronfern]

Feature (What you would like to be added):
Should be possible to scale up a single node etcd to a multi node etcd cluster

Motivation (Why is this needed?):
With multi node being available as an alpha feature, it is possible that we want to convert an existing single-node etcd to be multi-node. We should be able to support this feature and allow this scaling up of etcd from a single member to multiple members

Approach/Hint to the implement solution (optional):
New members are to be added to the cluster as learners. Learners are non-voting members, but still receive all data from the leader. When a learner has caught up with leader’s data, it can be promoted to a voting member