FunctionApp:Setting LinuxFxVersion to custom image with 'az functionapp config' sets WEBSITES_ENABLE_APP_SERVICE_STORAGE to false

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**Describe the bug**
For a webapp, when we set LinuxFxVersion to a custom image url, we also seem to be setting 'WEBSITES_ENABLE_APPSERVICE_STORAGE' to false. The code is here: https://github.com/Azure/azure-cli/blob/ed1fe586d24b5d78127c0b616cd7e32b765d0bfc/src/azure-cli/azure/cli/command_modules/appservice/custom.py#L145-L149

**To Reproduce**

1. Take any existing function app and set WEBSITES_ENABLE_APP_SERVICE_STORAGE app setting to true.

2. Take the existing web/function app and run the az webapp config set command to update linuxfxversion to a custom/blessed functions image:

az webapp config set -g -n --subscription --linux-fx-version "DOCKER|mcr.microsoft.com/azure-functions/node:3.0.14916-node12-appservice"

3. Then see the app settings of the function app - WEBSITES_ENABLE_APP_SERVICE_STORAGE is now set to false.

**Expected behavior**
When updating the linuxfxversion of any existing web/function app, only the linuxfxversion should have changed. Mounting app service storage is supported in the backend for custom and blessed functions images. This hardcoded behaviour from the client side has unexpected consequences for functions customers, since they often set LinuxFxVersion to blessed functions images urls.

**Environment summary**

**Additional context**