[services-systemd] Follow the flexible example of openrc

- Make *mandatory* optional (and default to false); this
   allows shorter lists of entries
 - Allow degenerate entries which are just a name
   (which have *mandatory* set to false as well).

SEE #992

src/modules/services-systemd/main.py

@@ -35,18 +35,25 @@ def systemctl(targets, command, suffix):
     silently.
     """
     for svc in targets:
+        if isinstance(svc, str):
+            name = svc
+            mandatory = False
+        else:
+            name = svc["name"]
+            mandatory = svc.get("mandatory", False)
+
         ec = libcalamares.utils.target_env_call(
-            ['systemctl', command, "{}{}".format(svc['name'], suffix)]
+            ['systemctl', command, "{}{}".format(name, suffix)]
             )
 
         if ec != 0:
-            if svc['mandatory']:
-                return ("Cannot {} systemd {} {}".format(command, suffix, svc['name']),
+            if mandatory:
+                return ("Cannot {} systemd {} {}".format(command, suffix, name),
                         "systemctl {} call in chroot returned error code {}".format(command, ec)
                         )
             else:
                 libcalamares.utils.warning(
-                    "Cannot {} systemd {} {}".format(command, suffix, svc['name'])
+                    "Cannot {} systemd {} {}".format(command, suffix, name)
                     )
                 libcalamares.utils.warning(
                     "systemctl {} call in chroot returned error code {}".format(command, ec)

src/modules/services-systemd/services-systemd.conf

@@ -17,7 +17,12 @@
 #   - *mandatory* is a boolean option, which states whether the change
 #     must be done successfully. If systemd reports an error while changing
 #     a mandatory entry, the installation will fail. When mandatory is false,
-#     errors for that entry (service or target) are ignored.
+#     errors for that entry (service or target) are ignored. If mandatory
+#     is not specified, the default is false.
+#
+# An entry may also be given as a single string, which is then
+# interpreted as the name of the service. In this case, mandatory
+# is also set to the default of false.
 #
 # Use [] to express an empty list.
 
@@ -44,7 +49,9 @@
 #     mandatory: false
 #
 # # Disables <name>.target
-# disable-targets: []
+# #     .. this shows how to use just the name
+# disable-targets:
+#   - graphical
 #
 # # Masks (stronger version of disable). This section
 # # is unusual because you **must** include the suffix