Fix instance-store registration using boto3 and add architecture config and cli option

[kvick]

So this should wrap up #252 and #249 with fixes to some problems when doing instance-store registrations and providing correct parameters. Surprisingly architecture, wasn't getting propagated correctly for instance store bakes (I kept thinking it should have been in context.base_ami.architecture), so decided to make it a default configuration and overridable via the finalizers per @bmoyles feedback.

The Caveat Emptor here is that this will now happily allow you to bake on x86_64 and register it as i386