Bug 1538209

Description of problem:

In the documentation here - 
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/12/html-single/advanced_overcloud_customization/index#defining_a_composable_network

The description of how to use process_templates.py in conjunction with network_environment.yaml is a bit unclear as this tool won't generate a network_environment.yaml file, just the parameters which can be used in the file.

This is some feedback we have gotten from the field:
./tools/process_templates.py does render many files, but *not* network-environment.yaml, because there is no j2 for that. The whole note implies "just make your network_data.yaml, use the tool and you will have a ready to use network-environment.yaml". This will not work. And this note is *much* to short to understand the overall process. In https://access.redhat.com/solutions/3299211 we have much cleaner and better explanation about the process and what's all about than in the doc. So better remove from the docu something too short to understand and for the mentioned case barely usable - or - rewrite this note to be a real chapter and explain the process properly.

I also have a problem with "local copy of /usr/share/openstack-tripleo-heat-templates/" because I would never every copy everything over and have hundreds of files where I am only changing 5-10. We have "--templates" to exactly avoid copying unneeded files somewhere to
- make things more readable
- easier to maintain
- easier to diff in case of upgrades
- easier to understand.

For sure you can copy over for rendering and afterwards pull out the needed files.