Add detected_arch column to Host

I'm scared of migrations.

The idea here is to allow arbitrary architecture values from Recog or import sources, while still enforcing some regularity. 

No verification steps, because what this really needs is code/design/sanity review.
This seems spurious to me.  If the `ARCHITECTURES` constant changes, there will be needless churn in the spec.  This test is intended to describe the behavior that all values in the exposed constant are valid.