Update LifecycleEnvironment field.

`entities.LifecycleEnvironment` has a `prior` foreignkey that points to
another instance of a `entities.LifecycleEnvironment`, which in turn is
renamed to `prior_id` before any actions can be performed. However, the
`Katello` API expects that this attribute will remain unchanged, so this
PR makes sure that `prior` will remain this named to avoid issues.
This might be a bit more concise?

```python
lc_attrs = super(LifecycleEnvironment, self)._factory_data()
lc_attrs['prior'] = lc_attrs.pop('prior_id')
return lc_attrs
```

I'm nitpicking, though, and your code works just fine as is.