To avoid confusion between the two servers, rename the
daos_io_server to daos_engine (data plane) and keep daos_server
for the control plane.
Try to support both "engines:" and "servers:" in the yaml
file for backward compatibility.

Signed-off-by: Tom Nabarro <tom.nabarro@intel.com>
Co-authored-by: Johann Lombardi <johann.lombardi@intel.com>