Remove extra bookeeping for worker nodes

[neoaggelos]

Summary

Remove extraneous worker nodes table.

Changes

This table is used as bookkeeping for cluster worker nodes, but only causes issues right now:

• If a worker node has already left the cluster, it is impossible to remove it, as the client.DeleteNode() will fail.
• If a worker node fails to join, then it still gets an entry on the worker_nodes table, therefore is unable to join with that name (with a not useful error message that the node already exists).

Also, we begin marking nodes with k8sd.io/role=control-plane or k8sd.io/role=worker, such that we know these nodes are managed by k8sd

[bschimke95]

LGTM, so in essence you moved the "bookkeeping" from k8sd to the workers themselves, right?