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.....

Also adding tempest in allowlist_externals as it is required
by unit test also.

Fixing tox.ini for tox4 changes.

Change-Id: I55cd9855d8232c007b012c19755c837e97823aea
diff --git a/tox.ini b/tox.ini
index 0b793f0..c784293 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,7 +1,6 @@
 [tox]
 envlist = pep8,py39,bashate,pip-check-reqs
 minversion = 3.18.0
-skipsdist = True
 ignore_basepython_conflict = True
 
 [tempestenv]
@@ -24,7 +23,22 @@
     OS_STDERR_CAPTURE=1
     OS_TEST_TIMEOUT=160
     PYTHONWARNINGS=default::DeprecationWarning,ignore::DeprecationWarning:distutils,ignore::DeprecationWarning:site
-passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH TEMPEST_CONFIG TEMPEST_CONFIG_DIR http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY ZUUL_CACHE_DIR REQUIREMENTS_PIP_LOCATION GENERATE_TEMPEST_PLUGIN_LIST
+passenv =
+    OS_STDOUT_CAPTURE
+    OS_STDERR_CAPTURE
+    OS_TEST_TIMEOUT
+    OS_TEST_LOCK_PATH
+    TEMPEST_CONFIG
+    TEMPEST_CONFIG_DIR
+    http_proxy
+    HTTP_PROXY
+    https_proxy
+    HTTPS_PROXY
+    no_proxy
+    NO_PROXY
+    ZUUL_CACHE_DIR
+    REQUIREMENTS_PIP_LOCATION
+    GENERATE_TEMPEST_PLUGIN_LIST
 usedevelop = True
 allowlist_externals =
     find