[RFE] Support for custom data (or document why it is not supported)

[anhvoms]

Current situation

azure-init does not do anything with custom-data. See this article for custom-data usage on Azure.

Impact

Unlike user-data, custom-data is only available during provisioning phase because it's only available in the ovf-env.xml file. If the provisioning agent doesn't take action on custom-data, it will not be available again.

Ideal future situation

Provide some options for how customers want to handle custom-data. A couple options

1. We save custom-data to a file and the customer chooses what to do with it.
2. We assume it's a script and execute custom-data like a script.
3. Detect that custom-data exists and print a WARNING message to recommend that customer use user-data instead.

**Implementation options

[ Optional: please provide one or more options for implementing the feature requested ]

Additional information

Differences between custom-data and user-data: custom-data is only available to privileged users as it's only available in the ovf-env.xml in a limited window during provisioning (the ovf-env.xml is only available through mounting of the provisioning ISO, which is a privileged action). user-data is available throughout the lifetime of the VM, and is accessible by anyone who has access to the VM. user-data can be updated throughout the lifetime of the VM while custom-data cannot be updated.