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

site.yml

@@ -27,6 +27,23 @@
   - init
   - config
   - update
+- hosts: k3s_cluster
+  roles:
+  - role: longhorn-compatible
+  tags:
+  - init
+  - add-server
+  - add-agent
+  - longhorn-compatible
+- hosts: localhost
+  gather_facts: no
+  roles:
+  - role: longhorn
+  tags:
+  - init
+  - add-server
+  - add-agent
+  - longhorn
 - hosts: localhost
   gather_facts: no
   roles: