Cap sphinx for py2 to match global requirements
MODIFY
doc/requirements.txt
doc/requirements.txt
openstackdocstheme>=1.18.1 # Apache-2.0
reno>=2.5.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
sphinxcontrib-httpdomain>=1.3.0 # BSD

Cap sphinx for py2 to match global requirements

Change-Id: Icbc1b3a92f796bf9e3186a512dd21661db035583