Restructured and updated Neutron network node section

As part of the installation guide improvement project, I performed
the following operations on the Neutron network node section:

1) Removed modularity since we will only support the ML2 plug-in
   for Icehouse.
2) Added some <procedure> tags to separate content. Future patches
   will add descriptions of each procedure.
3) Unified indentation and spacing.
4) Removed *.ini configuration steps since Icehouse doesn't require
   editing *.ini files.
5) Moved 'auth_uri' key under [keystone_authtoken] section in
   neutron.conf.
6) Removed defunct reference in Neutron use cases section.

This section should continue to work for installations using the Open
vSwitch plug-in for Neutron. Future patches will update the content
to use ML2.

Change-Id: I23e0ba8df56ef3227443b5588ccb51325ec649dd
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements
Merge "Restructured and updated Neutron network node section"