fix: Do not require 'data' physical interfaces (#86)
fix: Do not require 'data' physical interfaces (#86)

The 'physical_interfaces: data:' list in the config file is optional.
When it is not included the 'config.py' method to get data interfaces
should returns empty list and the method to get the list size returns
'0'.
open-power-ref-design-toolkit/cluster-genesis,open-power-ref-design-toolkit/cluster-genesis