Pin docutils version

The 0.13.1 release breaks some documentation jobs when it tries to build
docs with remote images. This has been reported upstream[0]. The version
matches the supported docutils versions for sphinx.

The requirements patch is here: [1]

[0] https://sourceforge.net/p/docutils/bugs/301/
[1] https://review.openstack.org/#/c/409529/

Change-Id: I13832c4103f5c2a3fd9271e1ed55598d786f6bde

test-requirements.txt

@@ -5,5 +5,6 @@
 bashate>=0.2 # Apache-2.0
 hacking<0.11,>=0.10.0
 
+docutils>=0.11,!=0.13.1  # OSI-Approved Open Source, Public Domain
 sphinx!=1.3b1,<1.4,>=1.2.1 # BSD
 oslosphinx>=4.7.0 # Apache-2.0