Support for ApplicationInsights

[giuliov]

Permit logging on AI as alternative to built-in logging.

Reason

The built-in logging is useful for testing with light workloads but is not intended for high-load production use. For production monitoring, Application Insights is recommended. If built-in logging is used in production, the logging record may be incomplete due to throttling on Azure Storage.
https://docs.microsoft.com/en-us/azure/azure-functions/functions-monitoring#disable-built-in-logging

Foreseeable changes
ARM template creates AI resource
Configuration command to switch between modes by removing adding these keys:

• AzureWebJobsDashboard connection string to storage created by template
• APPINSIGHTS_INSTRUMENTATIONKEY key of AI
  Do not forget to expose logging to rules

Open issues
One AI resource per Instance or one per RG or what?