Rke: Run RKE on a single node to add to the cluster

[HighwayofLife]

RKE currently runs against your entire cluster, but when the desire is to add or cleanup (remove) a single node in that cluster, it is unnecessary to run RKE against every node that exists.
Would be nice to have the ability to run rke in a running cluster to add a single node. This might have to apply only to worker nodes if the etcd or control planes all need to be aware of each other.

[superseb]

@galal-hussein You added update-only for updating workers only, right?

+       cli.BoolFlag{
+           Name:  "update-only",
+           Usage: "Skip idempotent deployment of control and etcd plane",
+       },

[galal-hussein]

@HighwayofLife @superseb yes using update-only will only update the workers and should skip running rke on control or etcd.

[HighwayofLife]

That gets halfway to my request, if you only need to add a single node, and you're running 10 workers, RKE will still run against all 11 nodes even with update-only.

[willmao]

it is quite useful for rke to add/remove single node to the cluster, if one node is down and unreachable, rke up --update-only will fail, in this case, we want to remove the dead node and add other node to replace it.

it is also useful when cluster auto-scaling.