Add longhorn distributed storage to the k3s-cluster

This change adds longhorn, an addition to Kubernetes that adds the
ability to use distributed storage over all nodes to the cluster.

Note, that I tried that in December already but due to very high load on
the machines I rolled _everything_ back. Though, it turned out that the
high load was not because of longhorn, but instead because of bad
configuration of the server, as described in the see-also commit.

Reference: https://longhorn.io/
Reference: https://longhorn.io/docs/1.10.1/deploy/install/install-with-helm/
See-also: 4b8a3d12c4 Use etcd instead of sqlite for k3s-server

roles/longhorn/defaults/main.yml

@@ -0,0 +1,3 @@
+longhorn_state: present
+longhorn_namespace: longhorn-system
+longhorn_chart_version: 1.10.1