add the ability to mask systemd units

Change-Id: I4f2a1ed3f5b2d4491784bef9b6e4c9b2f3896396

promenade/schemas/HostSystem.yaml

@@ -22,6 +22,8 @@ data:
           type: boolean
         stop:
           type: boolean
+        mask:
+          type: boolean
       additionalProperties: false
     apt_source_line:
       type: string

promenade/templates/include/up.sh

@@ -127,7 +127,7 @@ log === Starting Docker and Kubelet ===
 set -x
 systemctl daemon-reload
 
-{% for a in ['enable','start','stop','disable'] %}
+{% for a in ['enable','start','stop','disable','mask'] %}
 {% for u in config.get_units_by_action(a) %}
 systemctl {{ a }} {{ u }}
 {% endfor %}