Azure temporary disk not present for instance type

[Terry Cain]

Cloud Provider: Azure
Instance type: Dsv4

The new Dv4, Dsv4, Ev4, and Esv4 instance types don't come with a temporary disk at /dev/disk/cloud/azure_resource

So when I create/reboot one of these new instances I get longer reboot times as cloud-init is waiting 2 minutes for this disk path to appear.
Looks like the function that looks for the disk is here - https://git.launchpad.net/cloud-init/tree/cloudinit/sources/DataSourceAzure.py?id=93cebe009d116230850c770227e9ead5c490c0d0#n888

Log snippet:
```
2020-10-21 12:34:43,120 - handlers.py[DEBUG]: start: init-network/activate-datasource: activating datasource
2020-10-21 12:34:43,120 - handlers.py[DEBUG]: start: azure-ds/activate: activate
2020-10-21 12:34:43,120 - handlers.py[DEBUG]: start: azure-ds/address_ephemeral_resize: address_ephemeral_resize
2020-10-21 12:34:43,120 - handlers.py[DEBUG]: start: azure-ds/wait-for-ephemeral-disk: wait for ephemeral disk
2020-10-21 12:34:43,121 - util.py[DEBUG]: Azure ephemeral disk: Waiting up to 120 seconds for the following files: ['/dev/disk/cloud/azure_resource']
2020-10-21 12:36:43,160 - util.py[DEBUG]: Azure ephemeral disk: Still missing files after 120 seconds: {'/dev/disk/cloud/azure_resource'}
2020-10-21 12:36:43,161 - DataSourceAzure.py[WARNING]: ephemeral device '/dev/disk/cloud/azure_resource' did not appear after 120 seconds.
2020-10-21 12:36:43,161 - handlers.py[DEBUG]: finish: azure-ds/wait-for-ephemeral-disk: SUCCESS: wait for ephemeral disk
2020-10-21 12:36:43,161 - handlers.py[DEBUG]: finish: azure-ds/address_ephemeral_resize: SUCCESS: address_ephemeral_resize
```

I've looked in the instance metadata between a Dsv4 and Dsv3 and there is nothing to indicate if this temporary disk will be present.

More than happy to make a patch to look for the instance type and skip the call if its not needed.

Additional info:

Azure temporary disk VM list -
https://docs.microsoft.com/en-us/azure/virtual-machines/azure-vms-no-temp-disk#what-are-the-differences-between-these-new-vm-sizes-and-the-general-purpose-dv3dsv3-or-the-memory-optimized-ev3esv3-vm-sizes-that-i-am-used-to

[Chris Pressland (cpressland)]

How's that for timing, I was literally going to raise this today. +1 and all that.

[Johnson Shi (johnsonshi)]

We're (MSFT) taking a look at this right now and seeing if anything can be done.

[Johnson Shi (johnsonshi)]

WIP PR addressing this issue at https://github.com/canonical/cloud-init/pull/716

[Eduardo Otubo (otubo)]

Red Hat QA also got this bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1925213

[Dan Watkins (oddbloke)]

This bug is believed to be fixed in cloud-init in version 21.1. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3797