ofagentapp: add cmd line option for datapath id.

Adding new command line option for providing datapath id to
differentiate agents when talk to controller.
  -i, --dpid=DATAPATHID      Specify Datapath ID.
Default value:
  DATAPATHID = 0x000000000000DA7A

Signed-off-by: Vitaliy Ivanov <vitaliyi@interfacemasters.com>