Developer documentation

* Turns TESTING into a rst file, that we include in the developer
  documentation, for instructions on how to run the unit tests.
* Link to a Vagrant project that sets up Neutron inside a VM.
* Adds a section for how to debug with Nose
* Add new section for Neutron Internals
* Neutron L2 Agent documentation - currently only OVS
* Make the Security Group API extension an example of how an API
  extension is implemented

Implements bp developer-documentation

Change-Id: I9b452abc9da3b1a41ae65cff727967de0eab12fe
Merge "Developer documentation"