replace iso_checksum with iso_checksum_url

Merged Sven-Hendrik Haase requested to merge github/fork/kBite/eliminate-hardcoded-hash-value into master

Created by: kBite

By replacing iso_checksum with iso_checksum_url there is no need to hardcode hash values into vagrant.json.

Trying to add mirror variable leeds to errors: vars inside variables are not reusable. See packer issue #874.

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