shill: vpn: Implement NSS database interface class.

Use the NSS class to lookup NSS certificates for OpenVPN.

BUG=chromium-os:28792
TEST=unit tests

Change-Id: I2e0c7924d664f375f5b01bc73506e2b91e6f8720
Reviewed-on: https://gerrit.chromium.org/gerrit/19457
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/openvpn_driver.h b/openvpn_driver.h
index 74a156f..7d657de 100644
--- a/openvpn_driver.h
+++ b/openvpn_driver.h
@@ -30,6 +30,7 @@
 class EventDispatcher;
 class Manager;
 class Metrics;
+class NSS;
 class OpenVPNManagementServer;
 
 class OpenVPNDriver : public VPNDriver,
@@ -157,6 +158,7 @@
   KeyValueStore args_;
   Sockets sockets_;
   scoped_ptr<OpenVPNManagementServer> management_server_;
+  NSS *nss_;
 
   VPNServiceRefPtr service_;
   scoped_ptr<RPCTask> rpc_task_;