Fix Tempest Provisioning

- Use 'CONFIG_PROVISION_CIRROS_URL' instead of
  'CONFIG_PROVISION_TEMPEST_CIRROS_URL' which doesn't exist.

Change-Id: Iecc560cddb45dc3fc9c5b74dd852304e5ea19390
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>