Add artifact_checksum to drivers

[dadgar]

Add an artifact_checksum that using the go-getter checksum feature: https://github.com/hashicorp/go-getter/pull/1/files

[armon]

This may not be necessary after talking to @mitchellh, since the checksum can be a query parameter in artifact_source

[catsby]

Maybe we could support both? An explicit checksum type config option feels better to me, but I'm fine either way.

Regardless, I'll clean up the Qemu driver that does this on it's own, since it's now built into go-getter, and I'll make sure the other drivers behave as expected when a checksum is included

[dadgar]

I could go either way. As long as we have a section in the docs documenting that you can add the checksum it is fine.

[catsby]

Proposing #381 as a fix

[dadgar]

#381 fixes

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