Skip Django related models generation when primary key is provided

It would be nice to skip generation of related Django model when related key is provided. Example:

```
class Author(models.Model):
...

class Book(models.Model):
author = models.ForeignKey(Author)
...
# don't generate extra author instance
mixer.blend(Book, author_id=3)
```
##

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Start at the Django model-generation path and reproduce the example with Book and author_id=3. The work is done when supplying the related model's primary-key field prevents Mixer from generating an extra related Author instance.

Written by the indexing model from the issue text.