parse_config: optional strict mode

parse_config: optional strict mode

when passing a config from one cluster to another, we want to be strict
when parsing - it's better to fail the migration early and upgrade the
target node instead of failing the migration later (when significant
work for transferring disks and/or state has already been done) or not
at all, but silently lose config settings that the target doesn't
understand.

this also might be helpful in other cases - e.g. when restoring from a
backup.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>