User configurable dpid

"datapath_id" (dpid) is used to identify the logical_device in ONOS. "dpid" is something voltha generates on its own and not really derived from the OLT device (for ex: using mac-address).

If there is a need to have a certain network configuration for a dpid in advance, like for example I want to associate a certain OF driver with the dpid, it cant be done with the present voltha, because of the dynamic nature of generation of dpid.

It should be possible to know in advance the dpid a OLT gets in voltha. This will be useful in cases where we need to have associate certain network configuration in ONOS for a device in advance. It should be possible for the user to configure the dpid that will be used.