tox: add /bin/bash to whitelist_externals
tox: add /bin/bash to whitelist_externals

Since tox 1.5, commands outside the virtual environment must be
explicitly listed in whitelist_externals. Without that, some commands
emit the warning:

    WARNING:test command found but not installed in testenv
       cmd: /bin/bash
       env: ...
    See also the whitelist_externals envconfig setting.

Change-Id: Iad1f623defbb32dafb5bad3613cec7a8409e2b2c

gerrive/horizon,noironetworks/horizon,coreycb/horizon,yeming233/horizon,django-leonardo/horizon,wolverineav/horizon,openstack/horizon,Tesora/tesora-horizon,NeCTAR-RC/horizon,Mirantis/mos-horizon,coreycb/horizon,bac/horizon,wolverineav/horizon,Tesora/tesora-horizon,wolverineav/horizon,Mirantis/mos-horizon,NeCTAR-RC/horizon,noironetworks/horizon,openstack/horizon,redhat-openstack/horizon,noironetworks/horizon,redhat-cip/horizon,django-leonardo/horizon,Tesora/tesora-horizon,bigswitch/horizon,bac/horizon,bigswitch/horizon,noironetworks/horizon,gerrive/horizon,NeCTAR-RC/horizon,sandvine/horizon,django-leonardo/horizon,BiznetGIO/horizon,redhat-openstack/horizon,django-leonardo/horizon,yeming233/horizon,dan1/horizon-x509,sandvine/horizon,Mirantis/mos-horizon,davidcusatis/horizon,sandvine/horizon,redhat-cip/horizon,redhat-openstack/horizon,ChameleonCloud/horizon,bigswitch/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,gerrive/horizon,dan1/horizon-x509,redhat-openstack/horizon,coreycb/horizon,dan1/horizon-x509,dan1/horizon-x509,gerrive/horizon,Tesora/tesora-horizon,davidcusatis/horizon,sandvine/horizon,wolverineav/horizon,bac/horizon,yeming233/horizon,Mirantis/mos-horizon,NeCTAR-RC/horizon,redhat-cip/horizon,bac/horizon,ChameleonCloud/horizon,openstack/horizon,davidcusatis/horizon,coreycb/horizon,ChameleonCloud/horizon,redhat-cip/horizon,davidcusatis/horizon,bigswitch/horizon,openstack/horizon,BiznetGIO/horizon,yeming233/horizon,BiznetGIO/horizon