Refactor diagrams and update user guide

[jputrino]

@mattgreene @richbrowne

What issues does this address?

Fixes #102 #80

What's this change do?

-updates diagrams in the user guide for clarity and adherence to F5 branding -some text edits for clarity and accuracy -clarified 'agent redundancy'

Where should the reviewer start?

Build the docs and view the images inline (we can do this together or I can provide instructions).

Any background context?

[mattgreene]

docs/media/f5-lbaas-horizontal-scaleout.png

Need to add a line connecting plugin driver to the external agent via the message queue.

[mattgreene]

docs/_build/map_multiple-agents-same-host.html

step 6. Need a space between in "sudoervice"

[mattgreene]

docs/_build/map_bigip-ha.html

Clustering is coming soon.

[mattgreene]

Need to tweak the headings. Most of the "User Guide" is nested under the "Compatibility" section. Need to move it up a level. How about:

Preface (Heading 1) Release Version (Heading 2) Compatibility (Heading 2) User Guide (Heading 1) Overview (Heading 2) ...

[mattgreene]

Address these minor comments and I think we're good for the set of change.

[jputrino]

@mattgreene - should be good to go!