Allow setting theme name in Channel fixtures

[gorkalaucirica]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Related tickets	no
License	MIT

This PR enables the possibility to set the theme name in the fixtures as follows:

sylius_fixtures:
    suites:
        default:
            listeners:
                orm_purger: ~
                logger: ~

            fixtures:
                channel:
                    options:
                        custom:
                            us_web_store:
                                name: US Web Store
                                locales:
                                    - "%locale%"
                                currencies:
                                    - "%currency%"
                                shipping_methods:
                                    - ups
                                    - dhl_express
                                    - fedex
                                payment_methods:
                                    - cash_on_delivery
                                    - bank_transfer
                                theme_name: vendor/theme-name

[pamil]

👍

[pjedrzejewski]

Thanks Gorka! :)