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-compatible/tasks/main.yml

@@ -0,0 +1,22 @@
+- name: Ensure longhorn dependencies are installed
+  package:
+    state: present
+    name:
+    - open-iscsi
+    - nfs-common
+
+- name: Ensure iscsid service is enabled and started
+  ansible.builtin.systemd_service:
+    name: iscsid
+    enabled: true
+    state: started
+
+- name: Ensure iscsi_tcp module is added
+  community.general.modprobe:
+    name: iscsi_tcp
+    state: present
+
+- name: Ensure nfs module is added
+  community.general.modprobe:
+    name: nfs
+    state: present