Replace images with generated sphinx diagrams

[Jesse Pretorius]

The current images we use in the documentation are very pretty, but require the right commercial application to edit.

We should make use of sphinx extensions to replace them and make them easier to maintain.

The following extensions are already in OpenStack global requirements, and therefore are easy to add and take advantage of:
http://blockdiag.com/en/blockdiag/sphinxcontrib.html
http://blockdiag.com/en/seqdiag/sphinxcontrib.html

The following extensions are not in OpenStack global requirements, and therefore will require a patch there (or we could use an alternative pinning method):
http://blockdiag.com/en/nwdiag/sphinxcontrib.html
http://blockdiag.com/en/actdiag/sphinxcontrib.html

Using these extensions can allow us to wholly replace the images with less pretty, but more maintainable diagrams and flow charts.