voltha core changes to support OLT Reboot

rw-core should adhere to the corresponding behaviour
RW-core:
• On setting OLT device conn_state as “UNREACHABLE”, core’s device_state_transitions graph
should trigger handler for,

1. Disable all child devices via DisableAllChildDevices (should not be done, but it does not harm)
2. Delete UNI ports associated with all child devices via DeleteAllUNILogicalPorts. (should happen as an outcome of DeleteAllChildDevices)
3. Forcefully remove the flows (on the logical device as well as the device tables) associated with each of the ONUs; do not pass via route computation as that will fail.
4.
5. Delete all child devices via DeleteAllChildDevices.(these remove references of all child devices in ONU adapter)
6. Remove ONU devices entry from the core model (should happen as an outcome of DeleteAllChildDevices).
7. REMOVE the Logical NNI port.
8. Disconnect logical device associated with device. Core ensures that this is done onl after all the logical ports are REMOVED
9.
(Eventually, flows associated with the ONU devices will be cleared from core model.)