shill: vpn: Provide OpenVPN.Mgmt.Enable and VPN.Domain

Neither of these properties are used in shill (not used in flimflam
either), but due to its fleeting use in a test script, the practice
of setting these parameters became a part of Chrome/ONC.  Support
setting these parameters for now, while we're in transition.

BUG=chromium-os:29286
TEST=Rerun unit tests

Change-Id: Id6abffd964d266cc06e67432a9cd938cbaba136a
Reviewed-on: https://gerrit.chromium.org/gerrit/19986
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
diff --git a/vpn_service.h b/vpn_service.h
index 6466a57..a7f0544 100644
--- a/vpn_service.h
+++ b/vpn_service.h
@@ -48,6 +48,9 @@
   std::string storage_id_;
   scoped_ptr<VPNDriver> driver_;
 
+  // Provided only for compatibility.  crosbug.com/29286
+  std::string vpn_domain_;
+
   DISALLOW_COPY_AND_ASSIGN(VPNService);
 };