Add API tests for path "architectures"

Add simple tests for `/architectures` and `/architectures/:id`. Again, simply
check whether it is possible to get the relevant page, create an entity, read
that entity after creation and delete it.

Turn class `robottelo.entities.Architecture` into a factory.
Rename argument/field "model" to "entity"

Do this for class `robottelo.orm.OneToManyField`. This change allows the
terminology used in this application to be more focused and correct.