Remove installation of tobiko in the doc/source/conf.py file

Docs configuration file doc/source/conf.py shouldn't install
dependencies, like tobiko project itself as it seems that
this causes failures of the docs builds on the readthedocs.

So this patch removes it from that file and adds sphinx_rtd_theme
to the docs requirements file.
It also moves installation of tobiko project to the tox.ini file to make
Zuul docs job happy.

Change-Id: I0b227f87ade2d9561dd8aee988a565cbef31c7ae

doc/readthedocs_requirements.txt

@@ -4,3 +4,4 @@
 -r ../requirements.txt
 -r ../extra-requirements.txt
 -r ./requirements.txt
+sphinx_rtd_theme>=0.5.1,<1    # Apache-2.0

doc/source/conf.py

@@ -56,13 +56,6 @@ release = get_version.get_version()
 version = '.'.join(release.split('.', 2)[:2])
 
 
-# -- Install requirements ----------------------------------------------------
-
-from tools import install
-install.pip_install('sphinx_rtd_theme>=0.5.1,<1')   # Apache-2.0
-install.install_tobiko()
-
-
 # -- General configuration ---------------------------------------------------
 
 # Add any Sphinx extension module names here, as strings. They can be

tox.ini

@@ -275,8 +275,9 @@ basepython = {[testenv:py3]basepython}
 envdir = {toxworkdir}/docs
 deps =
     {env:TOX_CONSTRAINTS}
-    -r{toxinidir}/doc/requirements.txt
-
+    -r{toxinidir}/doc/readthedocs_requirements.txt
+commands =
+    {envpython} -m pip install {env:TOX_CONSTRAINTS} {toxinidir}
 
 [testenv:linkcheck]
 
@@ -285,6 +286,7 @@ basepython = {[docs]basepython}
 envdir = {[docs]envdir}
 deps = {[docs]deps}
 commands =
+    {[docs]commands}
     sh -c 'cd {toxinidir}/doc/source && sphinx-build -W -b linkcheck . ../build/linkcheck'
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system