Wendelin.core 2 tests need to be able to mlock 64M to run.
Increase corresponding limit for all subprocessed spawned by cron, which
covers slapos -> supervisord -> ...

See added comments in slapos_limit.conf for why this has to be done in
/etc/security/limits.d/slapos.conf instead of
/etc/systemd/system/cron.service.d/override.conf

I testd this to work on a KVM of mine with manually putting
/etc/security/limits.d/slapos.conf in place. Before the patch:

    https://erp5.nexedi.net/test_result_module/20201009-341BCC3C/17
    (lots of "OSError: [Errno 1] Operation not permitted" for mlock requests)

After the patch:

    https://erp5.nexedi.net/test_result_module/20201009-6B97A93B/17
    (test.wcfs/fs passes correctly)

/cc @rafael, @tomo, @luke, @Nicolas