Add a venv that can generate/write/update the states diagram

Instead of requiring manual understanding of how to generate
the states diagram, add an explicit virtualenv that can be
used to create and generate that docs diagram (and replace the
existing file with it).

Change-Id: I1738da9bf08bfda9b9be01489e8456569a31709e

tools/states_to_dot.py

@@ -22,8 +22,6 @@ top_dir = os.path.abspath(os.path.join(os.path.dirname(__file__),
                                        os.pardir))
 sys.path.insert(0, top_dir)
 
-# To get this installed you may have to follow:
-# https://code.google.com/p/pydot/issues/detail?id=93 (until fixed).
 import pydot
 
 from ironic.common import states

tox.ini

@@ -18,6 +18,11 @@ commands =
 [tox:jenkins]
 downloadcache = ~/cache/pip
 
+[testenv:genstates]
+deps = {[testenv]deps}
+    pydot2
+commands = {toxinidir}/tools/states_to_dot.py -f {toxinidir}/doc/source/images/states.png
+
 [testenv:pep8]
 commands =
   flake8 {posargs}