Don't run app config initializer for generator

[andyw8]

In #1097 a change was made to require that the Shopify environment variables were present. As this code is in an initializer, it also runs when the generators run, but at that point in time the environment variables may not yet be available (e.g. because dotenv-rails hasn't been added yet), causing an error.

Based on a suggestion in https://stackoverflow.com/a/32789186, we can check if the initializer is running from a generator or not.

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in docs/, if necessary
• For security fixes, the Disclosure Policy must be followed.

[andyw8]

Rather than copy the exact updated template code into the test, I've simplified this to check for just the 'essential' part. Verifying the template code doesn't prove that the generator works – ideally we should generate an app as part of the test suite.

[xiaoboa]

@andyw8 with this change, the reference of ShopifyApp.configuration in generator code(not template code) should also be removed, there is one at home_controller_generator.rb

      def embedded_app?
        ShopifyApp.configuration.embedded_app?
      end

the method embedded_app? is no longer needed any more.
Otherwise the generated code of HomeController by default will be an authenticated home_controller instead of the unauthenticated home_controller, which is conflict with the README.md

By default, this generator creates an unauthenticated home_controller and a sample protected products_controller.

[paulomarg]

That's a good point @xiaoboa, I'm actually investigating that myself. We still need need the method if we're creating non-embedded apps but I'll make sure that home_controller is not authenticated.

Thanks!