Don't fail if there is no nf_conntrack_proto_gre module available

It may be that it is already compiled in the kernel so there is no
need to load kernel module in such case.

Change-Id: Ie1d32e3fd529e13958857cb3ced6710eebde1e4d
diff --git a/lib/neutron_plugins/ovs_source b/lib/neutron_plugins/ovs_source
index 294171f..08951d1 100644
--- a/lib/neutron_plugins/ovs_source
+++ b/lib/neutron_plugins/ovs_source
@@ -211,5 +211,5 @@
 
 # load_conntrack_gre_module() - loads nf_conntrack_proto_gre kernel module
 function load_conntrack_gre_module {
-    sudo modprobe nf_conntrack_proto_gre
+    load_module nf_conntrack_proto_gre False
 }