Currently, if the inspection network is not provided, neutron-based
network interfaces fail with something like:
Driver redfish does not support inspection (disabled or not implemented)
This is utterly misleading. Use a hand-crafted error message instead.
Same for the PXE boot interface. Also add missing documentation.
Change-Id: I79086db1c270e02a6c74b870acc336e8da54dea3
018a7dcaed