Fix tox4 issues

New tox version 4 has changed interpretation of tox.ini

The passenv parsing requires the list to be comma-separated.
Since the names are case-insensitive, duplicates are removed.

skipsdist is now a synonym of no_package and avoids the
installation of the package in all cases.
However, the docs environment requires it (for cliff's auto
generation).

Change-Id: Ia7292c4c72d01ddd5320675b052a27bfbf7ea974

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.1.1
 envlist = pep8,py3
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -13,7 +12,7 @@ deps =
   -r{toxinidir}/test-requirements.txt
 commands =
   stestr run {posargs}
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY OS_DEBUG GENERATE_HASHES
+passenv = HTTP_PROXY, HTTPS_PROXY, NO_PROXY, OS_DEBUG, GENERATE_HASHES
 
 [testenv:linters]
 skip_install = True
@@ -54,12 +53,15 @@ commands =
 
 [testenv:docs]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-deps = {[testenv:docs]deps}
+skip_install = True
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html