r/collectd-sensors: Install collectd sensors plugin

The *sensors* plugin for collectd reads temperature information from the
I²C/SMBus using *lm_sensors*.  Naturally, it is only useful on physical
machines, so it is not installed or enabled by default.

roles/collectd-sensors/tasks/main.yml

@@ -0,0 +1,8 @@
+- name: ensure collectd-sensors is installed
+  package:
+    name: collectd-sensors
+    state: present
+  tags:
+  - install
+  notify:
+  - restart collectd