Add bash to tox whitelist_externals

Stop triggering triggering a “not installed in virtualenv” every time
bash is called. Since we use this in all tempest based jobs, we have
these unnecessary warnings all over the place.

Change-Id: I8af53c0241dbcd656c274944338384be0f2f250d

tox.ini

@@ -9,6 +9,8 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 usedevelop = True
 install_command = pip install -U {opts} {packages}
+whitelist_externals = bash
+
 
 [testenv:py26]
 setenv = OS_TEST_PATH=./tempest/tests