remove clouds.yaml files when running clean.sh
remove clouds.yaml files when running clean.sh

There have been a few reported errors of folks running into
issues with old clouds.yaml files that linger around. Remove
these if the user runs clean.sh.

1. http://lists.openstack.org/pipermail/openstack-dev/2016-January/084283.html
2. https://ask.openstack.org/en/question/84825/glance-upload_image-fails-in-latest-devstack/
3. https://ask.openstack.org/en/question/86071/devstack-installation-failure/

Change-Id: I618ea8e27b49af360c905df85af06d9b1eef8407

samgoon/devstack,group-policy/devstack,nawawi/openstack,openstack-dev/devstack,bswartz/devstack,Mahito/devstack,dtroyer/devstack,mssumanth/devstack,richliu/devstack,richliu/devstack,bswartz/devstack,nawawi/openstack,dirkmueller/devstack,noironetworks/devstack,noironetworks/devstack,samgoon/devstack,openstack-dev/devstack,group-policy/devstack,bigswitch/devstack,dirkmueller/devstack,Mahito/devstack,dtroyer/devstack,bigswitch/devstack,mssumanth/devstack