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.
Any reason why the limit is 32?  On PostgreSQL, limited length string columns actually take longer to update as the length is enforced as an additional constraint that limitless string don't have to run.  Unlike MySQL, PostgreSQL has no concept of fast, in-row VARCHAR(size) fields, all VARCHAR fields are treated the same in PostgreSQL.