Whitelist bash in tox.ini for zuul

Avoid these lines in every output of the layout gate job with
whitelisting bash:

WARNING:test command found but not installed in testenv
  cmd: /bin/bash
  env: /home/aj/vcs/OpenStack/openstack-infra/project-config/.tox/zuul
Maybe you forgot to specify a dependency? See also the
whitelist_externals envconfig setting.

Also, order list of external commands alphabetically.

Change-Id: I4d1fe347c8006e14bc0fe3c61b3783e9039648b7

tox.ini

@@ -78,10 +78,11 @@ deps =
      jenkins-job-builder==1.6.1
      zuul
 whitelist_externals =
-  rm
-  mkdir
-  jenkins-jobs
+  bash
   find
+  jenkins-jobs
+  mkdir
+  rm
 commands =
   rm -rf {envdir}/tmp
   mkdir -p {envdir}/tmp/jobs