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 is a bad way to write these examples as you're using information from the class under test to test the class.  This is a scenario where the specs should be less DRY.  (i.e. you should explicitly hardcode the values from `described_class::ARCHITECTURES` into this file instead of reusing the Array on the class under test.)