Fix a warning when running `tox -e api-ref`
Fix a warning when running `tox -e api-ref`

When executing `tox -e api-ref`, the following warning appears.

    WARNING:test command found but not installed in testenv
      cmd: /bin/rm
      env: /Users/tl3438/oss/ceilometer/.tox/api-ref
    Maybe you forgot to specify a dependency? See also ...

This patch set adds the whitelist_externals to api-ref.

Change-Id: Ic663d229990c028888f0ecd7f7b85561ad618bf4

ityaptin/ceilometer,openstack/ceilometer,openstack/ceilometer,ityaptin/ceilometer