Added ML2 controller node section to Neutron chapter

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

1) Modified structure to retain existing OVS plug-in sections while
   adding ML2 plug-in sections.
2) Added ML2 plug-in controller node section.

This patch primarily covers content. Future patches may address
structure and/or appearance issues.

Change-Id: Ic6955316f51b3b3ada45e27e7a6d48bf6a815004
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements