ovs-docker: Ability to set the default gateway.

ovs-docker currently lets one add the ability to set the
IP address on an OVS interface. Ability to set the default
gateway too can be an useful addition. (orchestrators
who plan to use OVS interfaces can do this from a single
utility instead of multiple utilities)

Requested-by: Marvin Pascual <marvin@pascual.com.ph>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>