job registry: avoid injecting the section id unconditionally in configs

job registry: avoid injecting the section id unconditionally in configs

this can result in a broken config due to it getting written out on
write_config serialization, and if a plugin did not declare `id` as
an option it understood (none do currently), it would then fail the
next parse, far from ideal...

As the section ID is available already anyway we should probably just
drop this, but for now avoid rushed changes and just make it
conforming to section config semantics and check if the option is
actually understood by the respective section type we're working on.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>