Docs need to build without upper-constraaints

We are using openstackdocstheme >= 1.26.0 which is well beyong
what is in stable/pike upper-constraints.txt.

Change-Id: I2285d7dab0f82ce3a875cff7a8e6612a9c47cf6a
Signed-off-by: Dean Troyer <dtroyer@gmail.com>

tox.ini

@@ -69,6 +69,7 @@ commands =
 
 [testenv:docs] 
 basepython = python3
+install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/doc/requirements.txt
 commands =  
     rm -rf doc/build
@@ -77,6 +78,7 @@ whitelist_externals = rm
 
 [testenv:releasenotes]
 basepython = python3
+install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf releasenotes/build
@@ -91,8 +93,8 @@ commands = reno new {posargs}
 
 [testenv:api-ref]
 basepython = python3
-deps =
-    -r{toxinidir}/doc/requirements.txt
+install_command = pip install -U {opts} {packages}
+deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html