Fix plugin autodoc generation

Documentation for plugins wasn't being generated because the plugin
projects weren't being installed in the docs environment. Add them to
doc/requirements.txt to make this work.

Change-Id: Id9be39971110fd2eb4519a0582c9bf2514cdcacd
Story: #1735016
Task: #13825

Author: Eric Fried

doc/requirements.txt

@@ -6,3 +6,25 @@ reno>=2.5.0 # Apache-2.0
 sphinx!=1.6.6,!=1.6.7,>=1.6.5,<2.0.0;python_version=='2.7' # BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.5;python_version>='3.4' # BSD
 sphinxcontrib-apidoc>=0.2.0 # BSD
+
+# Install these to generate sphinx autodocs
+aodhclient>=0.9.0 # Apache-2.0
+gnocchiclient>=3.3.1 # Apache-2.0
+python-barbicanclient>=4.5.2 # Apache-2.0
+python-congressclient<2000,>=1.9.0 # Apache-2.0
+python-designateclient>=2.7.0 # Apache-2.0
+python-heatclient>=1.10.0 # Apache-2.0
+python-ironicclient>=2.3.0 # Apache-2.0
+python-ironic-inspector-client>=1.5.0 # Apache-2.0
+python-karborclient>=0.6.0 # Apache-2.0
+python-mistralclient!=3.2.0,>=3.1.0 # Apache-2.0
+python-muranoclient>=0.8.2 # Apache-2.0
+python-neutronclient>=6.7.0 # Apache-2.0
+python-octaviaclient>=1.3.0 # Apache-2.0
+python-rsdclient>=0.1.0 # Apache-2.0
+python-saharaclient>=1.4.0 # Apache-2.0
+python-searchlightclient>=1.0.0 #Apache-2.0
+python-senlinclient>=1.1.0 # Apache-2.0
+python-troveclient>=2.2.0 # Apache-2.0
+python-zaqarclient>=1.0.0 # Apache-2.0
+python-zunclient>=3.4.0 # Apache-2.0