Remove deprecated fields from desc.json

Remove exptected_return_code & expected_result_files from desc.json
Clean up readme about desc.json
Add "terminate_early" to desc.json schema and add override warning

Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>
Change-Id: Ib7f4c5365ccf58b3c6d1c7e11c4af6817931e6a2