shill: OpenVPNDriver: Accept OpenVPN.ExtraCertPEM

Accept, but not yet implement the "OpenVPN.ExtraCertPEM" property.
When OpenVPN for ChromiumOS is updated to a newer release, this
will be used to populate the "--extra-certs" parameter.

BUG=chromium:249363
TEST=Unit test

Change-Id: Id6d532aea7904c3412199a6020452198f5b5ae9b
Reviewed-on: https://gerrit.chromium.org/gerrit/58936
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/openvpn_driver.cc b/openvpn_driver.cc
index d56d2df..db668df 100644
--- a/openvpn_driver.cc
+++ b/openvpn_driver.cc
@@ -104,6 +104,7 @@
   { flimflam::kProviderTypeProperty, 0 },
   { kOpenVPNCaCertPemProperty, Property::kArray },
   { kOpenVPNCertProperty, 0 },
+  { kOpenVPNExtraCertPemProperty, Property::kArray },
   { kOpenVPNKeyProperty, 0 },
   { kOpenVPNPingExitProperty, 0 },
   { kOpenVPNPingProperty, 0 },