glusterfs service: Ensure log directory exists for glustereventsd.

Prevents glustereventsd failing at startup in case it starts
before glusterd has started (whose `preStart` would also
create the needed directory).

nixos/modules/services/network-filesystems/glusterfs.nix

@@ -194,6 +194,10 @@ in
 
       after = [ "syslog.target" "network.target" ];
 
+      preStart = ''
+        install -m 0755 -d /var/log/glusterfs
+      '';
+
       serviceConfig = {
         Type="simple";
         Environment="PYTHONPATH=${glusterfs}/usr/lib/python2.7/site-packages";