Enhance network-list to allow --fields

Add the ability to specify --fields, using any valid vields, when
doing a network-list. This code was shamelessly borrowed from
do_list(). This will allow crafting network-list queries which
mirror the functionality of 'nova-manage network list', which this
query was intended to replace.

Change-Id: I7ec7066350cdb979a64d20d14362b12e3ec76496
Merge "Enhance network-list to allow --fields"