shill: openvpn: Load should be exclusive

If a parameter cannot be loaded, it should be cleared.  Do not load
services inclusive of properties held over from local configuration
or another profile.

BUG=None
TEST=Rerun unit tests.

Change-Id: I4d61719075f9bc7627ae8a466914249c0774e5ea
Reviewed-on: https://gerrit.chromium.org/gerrit/18958
Commit-Ready: Paul Stewart <pstew@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/openvpn_driver.cc b/openvpn_driver.cc
index 1e92edf..f1574bd 100644
--- a/openvpn_driver.cc
+++ b/openvpn_driver.cc
@@ -544,6 +544,8 @@
         storage->GetString(storage_id, property, &value);
     if (loaded) {
       args_.SetString(property, value);
+    } else {
+      args_.RemoveString(property);
     }
   }
   return true;