Add hddtemp to common_software

* To see how high the hdd temperatures are if one gets a mail from
smartd

group_vars/all

@@ -3,13 +3,15 @@
 common_software:
   - git
   - vim
-  - htop
   - tmux
-  - iotop
-  - iftop
-  - ncdu
-  - dmidecode
-  - lm_sensors
+  # diagnostic for admins
+  - htop  # processes
+  - iotop  # disk IO
+  - iftop  # network IO
+  - ncdu  # nice disk usage
+  - dmidecode  # general hardware information
+  - lm_sensors  # cpu/gpu temperatures
+  - hddtemp  # HDD temperatures
 
 client_software:
   - gnuplot