Prevent the unexpected with nova-manage network modify.

"nova-manage network modify" can misinterpret arguments if the user
provides values where they are unexpected, specifically the
--disassociate-project and --disassociate-host options. If the user
provides a project or hostname value for either of these values, it will
be interpreted as a positional argument and modify handler may behave as
if other options were provided (e.g. project or host).

This change handles the presence of a project or host option along with
--disassociate-project or --disassociate-host as an error, printing a
message and exiting immediately with an error code. While there are
other approaches (e.g. perform the disassociate modifications while
ignoring the others), preventing unexpected or inconsistent results by
doing nothing seems safest.

Change-Id: Iff7817fe3ed19a4ff330b4029f9c558d3b405e9b
Merge "Prevent the unexpected with nova-manage network modify."