Add media gallery to product fixture with updated product spec

[jamescowie]

Allow products to be created with an image.

    Given the following products exist:
      | name | sku  | type_id | price | tax_class_id | image                                          | category_ids |
      | test    | test  | simple  | 10.00 | 2                  | features/assets/placeholder.png | 2                   |

[alistairstead]

The create method is getting very long. I think we should start to extract some methods for some of these additional setup steps

[jamescowie]

I have moved the asset creation into its own private method but I agree create is now getting long and doing more than just "create" I think we should look at what we can do to improve this.

[alistairstead]