sha256sum and md5sum to execute on the remote machine

[tarcinil]

A discussion in the Slack #inspec channel leads to the idea that md5sum and sha256sum should be done on the remote machine to minimize encoding issues as well as long executions (file is transferred on unknown connection speeds).

md5sum: https://github.com/chef/train/blob/master/lib/train/file.rb#L57
sha256sum: https://github.com/chef/train/blob/master/lib/train/file.rb#L65

Discussion:
Slack: https://chefcommunity.slack.com/archives/C1XB6U6MN/p1516743917000525
Github: inspec/inspec#1898 (comment)