Make sure external commands are allowed

... otherwise tox refuses executing these commands. Also, bumps the min
version so that the version which supports allowlist_externals is used.

Change-Id: I3d8fc840a31b4226bf4ef2b60ec618735a2da642

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18
 skipsdist = True
 envlist = pep8,py
 
@@ -17,7 +17,9 @@ commands = stestr run --slowest {posargs}
 commands = {posargs}
 
 [testenv:pep8]
-commands = ./run-flake8
+allowlist_externals =
+    bash
+commands = bash run-flake8
 
 [flake8]
 ignore = E125,H803
@@ -27,4 +29,6 @@ show-source = true
 [testenv:releasenotes]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
+allowlist_externals =
+    bash
 commands = bash -c tools/releasenotes_tox.sh