Add sphinxcontrib-blockdig for graph drawing

Images are often powerful tool to communicate when making a doc.
This PS hopely adds sphinxcontrib-blockdig to documentation tool.
Please give me feedback if this is right way to add it.

Some examples are:
https://docs.openstack.org/infra/zuul/user/gating.html

Change-Id: I7e8d8322df4bcc651e43e28d68ea5128660c592c

doc/source/conf.py

@@ -22,7 +22,8 @@ sys.path.insert(0, os.path.abspath('../..'))
 extensions = [
     'sphinx.ext.autodoc',
     'openstackdocstheme',
-    'oslosphinx'
+    'oslosphinx',
+    'sphinxcontrib.blockdiag'
 ]
 
 # autodoc generation is a bit aggressive and a nuisance when doing heavy

test-requirements.txt

@@ -1,4 +1,5 @@
 sphinx>=1.5.1,!=1.6.1 # BSD
+sphinxcontrib-blockdiag>=1.1.0
 oslosphinx>=4.7.0 # Apache-2.0
 pbr>=3.0.0 # Apache-2.0
-openstackdocstheme>=1.16.0 # Apache-2.0
+openstackdocstheme>=1.16.0 # Apache-2.0