Fix ovs-vsctl call in add_tunnel_port()

When a new GRE tunnel is created OVS agent calls ovs-vsctl
incorrectly. Command:

    sudo ovs-vsctl --timeout=2 --may-exist add-port br-tun gre-1

is produced instead of correct one:

   sudo ovs-vsctl --timeout=2 -- --may-exist add-port br-tun gre-1

Fixes bug 1227675

Change-Id: I010961658a37028e41621c4a529b5a14e7587c7f
Merge "Fix ovs-vsctl call in add_tunnel_port()"