Only install doc requirements if needed

We only need doc requirements for the doc jobs, so only install them
there.

Also, update docs job to not publish dirtrees.

Change-Id: I928f4e24970a680bf340ce15a08a4e72432fb0e4

tox.ini

@@ -8,7 +8,6 @@ usedevelop = True
 install_command = pip install {opts} {packages}
 deps=
       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
-      -r{toxinidir}/doc/requirements.txt
       -r{toxinidir}/requirements.txt
       -r{toxinidir}/test-requirements.txt
 commands=
@@ -46,14 +45,16 @@ basepython = python3
 commands = python setup.py test --coverage --coverage-package-name diskimage_builder --testr-args='{posargs}'
 
 [testenv:docs]
+deps= -r{toxinidir}/doc/requirements.txt
 basepython = python3
 whitelist_externals =
   rm
 commands =
   rm -rf doc/build
-  sphinx-build -W -b html doc/source doc/build/html {posargs}
+  sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html {posargs}
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 basepython = python3
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html