Fix tox4 error

tox.ini started failing with Tox4 which had some
incompatible changes. One is passenv where we need
to pass each value in newline otherwise, it fails with
error:

 failed with pass_env values cannot contain whitespace,
 use comma to have multiple values in a single line,
 invalid values found.....

Other failure is due to the skipsdist = True.

Fixing tox.ini for tox4 changes.

Change-Id: I42f12c6209fdf298488061617e12d8b32ee5f8aa

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 envlist = py39,py38,py36,pep8,docs
 minversion = 3.18.0
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -92,7 +91,9 @@ commands =
   stestr --test-path=./tacker/tests/functional/sol_kubernetes_multi_tenant run --slowest --concurrency 1 {posargs}
 
 [testenv:dsvm-compliance-sol-api]
-passenv = {[testenv]passenv} *_PROXY
+passenv =
+  {[testenv]passenv}
+  *_PROXY
 commands_pre =
   git clone https://forge.etsi.org/rep/nfv/api-tests.git
   git -C api-tests checkout 2.6.1-fix-plu
@@ -131,6 +132,8 @@ allowlist_externals = bash
 commands = python ./tools/check_i18n.py ./tacker
 
 [testenv:docs]
+allowlist_externals =
+  sphinx-build
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -W -b html doc/source doc/build/html