Silence tox warning

tox -e genconfig and tox -e pylint both will generate a warning like:
WARNING:test command found but not installed in testenv
  cmd: /bin/bash
  env: /home/aj/vcs/OpenStack/openstack/manila/.tox/genconfig
Maybe forgot to specify a dependency?

Whitelist bash to remove the warning.

Change-Id: Ibab38ce2308760c7089d5d3e298bfc642367e7b4
Merge "Silence tox warning"