core.checksum_jobs doesn't work for remotes

[efiop]

#2891 (comment)

[Suor]

The proper fix might require implementing something like config inheritance:

• look at remote section first
• look at core second
• use default

For particular options like checksum_jobs.

This will require making config smarter most probably. So it's a research now.

[gurobokum]

I'm starting to work on the issue