The spare pool failover patch changed the logic of the failover flow
and cause the flow diagram generation to only render part of the flow.
This patch updates the diagram generation to show the whole failover
flow.
Change-Id: I8dc49f0ca74054869ec42fee4236533acb24eb3a
Octavia extensively uses TaskFlow flows for orchestration.
To make it easier for developers to understand these flows,
this patch adds a mechanism for generating graphviz representations
of the key Octavia TaskFlow flows.
It also updates our tox docs task to generate this documentation.
This patch depends on a fix to the TaskFlow export_to_dot method.
Added into conf.py as first step of configuration
Closes-Bug: #1561063
Change-Id: I914e1c062b400148565def37ccf618b3d2ea2573
Depends-On: I99f87af0b2bed959fcb43ef611b3186e23bd9549