shill: vpn: Parse openvpn IP configuration options.

The options are pushed to shill over DBus through the .Task.notify service
method.

BUG=chromium-os:27056
TEST=unit tests

Change-Id: I98d7d49c275f49daba5a6dbe0af0878bf82038a6
Reviewed-on: https://gerrit.chromium.org/gerrit/17219
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/dhcp_config.h b/dhcp_config.h
index f8cf28d..e340080 100644
--- a/dhcp_config.h
+++ b/dhcp_config.h
@@ -27,6 +27,8 @@
  public:
   typedef std::map<std::string, DBus::Variant> Configuration;
 
+  static const int kMinMTU;
+
   DHCPConfig(ControlInterface *control_interface,
              EventDispatcher *dispatcher,
              DHCPProvider *provider,