Friendly functions for creating new fabrics and controllers

[cecille]

Tests commonly need to mint new fabrics and controllers. It's totally possible to do this, but it's a bit yucky. We should create some functions with friendly names to simplify this and make it more obvious what's going on

ie, something like "create_new_controller_on_new_fabric", "create_new_controller_on_existing_fabric"

Then we could move all the NewCertificateAuthority / NewFabricAdmin / NewController stuff over to use this.

include sensible defaults for vendor and fabric IDs.