Bug #916312: nova-manage network modify --network flag is inconsistent.
Bug #916312: nova-manage network modify --network flag is inconsistent.

Change nova-manage network modify --network to --fixed_range, to match
nova-manage network delete.

Also document both.

Change-Id: I3c03a33e9e3576393252d771eb4ce3353a7f2eaa

n0ano/ganttclient