single-node: Support single node setups

[m-ildefons]

Add support for single-node setups.

[bk201]

@m-ildefons harvester_cluster_nodes = 1 works for me, is there a reason to make the change?

[votdev]

@m-ildefons harvester_cluster_nodes = 1 works for me, is there a reason to make the change?

I am also using harvester_cluster_nodes: 1 to set up a one-node cluster.

As @albinsun already mentioned, this PR radically changes how the nodes are configured. In the end I don't care, but the changes should provide the default configuration of a 3 node cluster. Because the way the PR is at the moment I have to invest time to build my configuration for such a setup.

[votdev]

@m-ildefons Please try to add the NVMe feature somehow, i think it might be helpful.

[m-ildefons]

I'll open a new PR soon. I'm currently working to make multiple separate networks possible as well.

Unfortunately, this means that the way the nodes are configured will radically change, but I promise you the new way will be reasonable easy to deal with and the new features will be worth it.