BYOF linux app service plan function apps shouldn't specify image

Currently each client maintains a mapping of (language - image). Based on the functions_worker_runtime appsetting it picks the right image and includes it in the create site request.

This mapping should not be exposed to clients and should exist inside the RP. Only apps using byoc should have to specify the image information. For blessed images the image selection should be handled at server side.

Once we have this implemented WEBSITES_ENABLE_APP_SERVICE_STORAGE can automatically be added if linuxfxversion is specified.