Ironic configuration references enumerating code to find example values

Presently the sample configuration suggests for an operator to enumerate the endpoint with-in the code to identify possible configuration values. We introduced this with the network_interface option, however with driver composition working towards utilizing the same configuration options, we should improve this as telling an operator to enumerate code is unfriendly.

Example taken from ironic.conf.sample:

# A complete list of network interfaces present on your system
# may be found by enumerating the
# "ironic.hardware.interfaces.network" entrypoint.

We should improve this somehow to help operators understand what the possible driver options are. Bonus points if we can tell them what drivers appear to have dependencies present. ;)