If ipv4_address is not set, the current logic in inventory.py sets the
addressing_mode to dhcp which prevents network_data.json being
generated. This means that even if node_network_data is set which
overrides ipv4_address, you have to set it anyway to ensure that your
node_network_data gets written to the file. This patch fixes the logic
such that only if both of ipv4_address and node_network_data are not
set, then it assumes addressing_mode=dhcp.
Change-Id: Iab8e0a8422d9f33c8c5b1362bdaefabf6f39d52e
Closes-Bug: #1656335
b70ef9eae8