mongodb: Add initialisation service

The mongodb service runs as user mongodb, and therefore
the preStart-script has no permissions to set up mongodb
directories. This is solved by adding an initialisation
service that runs as root and just sets up the required
directories.

modules/services/databases/mongodb.nix

@@ -102,20 +102,28 @@ in
 
     environment.systemPackages = [ mongodb ];
 
+    systemd.services.mongodb_init =
+      { description = "MongoDB server initialisation";
+
+        wantedBy = [ "mongodb.service" ];
+        before = [ "mongodb.service" ];
+
+        serviceConfig.Type = "oneshot";
+
+        script = ''
+          if ! test -e ${cfg.dbpath}; then
+              install -d -m0700 -o ${cfg.user} ${cfg.dbpath}
+              install -d -m0755 -o ${cfg.user} `dirname ${cfg.logpath}`
+          fi
+        '';
+      };
+
     systemd.services.mongodb =
       { description = "MongoDB server";
 
         wantedBy = [ "multi-user.target" ];
         after = [ "network.target" ];
 
-        preStart =
-          ''
-            if ! test -e ${cfg.dbpath}; then
-                install -d -m0700 -o ${cfg.user} ${cfg.dbpath}
-                install -d -m0755 -o ${cfg.user} `dirname ${cfg.logpath}`
-            fi
-          '';
-
         serviceConfig = {
           ExecStart = "${mongodb}/bin/mongod --quiet --config ${mongoCnf}";
           User = cfg.user;