Changes

smdp_Tests: rename and document custom_tls_cert module parameters

The module parameters mp_use_custom_tls_cert and mp_custom_tls_cert_path
now only relate to ES9+, so let's add an appropriate prefix to make that
clear.

Let's also add an explanatory comment what those module parameters do.

Change-Id: Ie37da6986410b663378531f4f0d50d95b3068964
Related: SYS#7339