prevent autovivification of sectionconfig options

prevent autovivification of sectionconfig options

If, somehow, someone passes a config to check_config with keys set
that are not in the options for that type, this fixed check
lead to autovivification, meaning that any future calls to the same
worker had an additional option for that type which is not optional

this lead to a wrongfully deleting of entries when updating an entry of
a different type, since all entries of the original types suddenly
did not satisfy their required options and would not get parsed
by read_file anymore (thus missing when a successful write_file was done)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>