Undo changes to the warlock.model_factory API

[nathanielknight]

The changes introduced by commit 64a771d ("Allow
resolver to be set on the Model") added a new keyword argument to the
warlock.model_factory function. Due to the way Python handles
positional arguments, the change alters the way that existing code is
interpreted.

Before the change,

warlock.model_factory(schema_arg, snd_pos_arg)

would execute with

kwargs == {
    'base_class': snd_pos_arg,
    'name': None,
}

but after the change it's interpreted as

kwargs == {
    'resolver': snd_pos_arg,
    'base_class'=model.Model,
    'name': None,
}

This commit re-arranges the arguments to restore the previous behaviour
in an effort to not break existing code.