VOL-288: Minor fixes for xPON cli in VOLTHA
Fix the following issues detected in the xPON cli:
* Missing validation for device ID
* 'show' command to display v_ont_ani, ont_ani
and v_enet does not work for specific device id
VOL-290: Use interface_stack to implement child-parent relationship for xPON interface objects
Move to an elegant approach to use child-parent relationship for xPON interface objects to
determine the OLT and ONU device id(s) for sending configuration
Change-Id: Ia26154e3fe1fba1a650c6a53f4d5232fb7369ce3