Fix warning 'test command found but not installed in testenv'
Fix warning 'test command found but not installed in testenv'

Add 'find' to whitelist_externals in tox.ini to prevent
warning: 'WARNING:test command found but not installed in testenv'.

Change-Id: I8cfbbea21872b79234b6a0046e9428d80f1f9b19

openstack/python-searchlightclient,openstack/python-searchlightclient