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.
You need to add `public` here do that the `Metasploit::Concern.run(self)` executes with `public` active and not `private`.