Azure: mount the default provisioning iso location before attempting to find it through device listing

[anhvoms]

Proposed Commit Message

Mount the default provisioning iso location before attempting to find it through device listing

With a few exceptions, Azure VM deployments receive provisioning metadata 
through the provisioning iso presented as a cdrom device (/dev/sr0).
The existing code attempts to find this device by calling blkid to find all devices
that have either type iso9660 or udf. This can be very expensive if the VM has
a lot of disks. This commit will attempt to mount the default iso location 
first and only tries to use blkid to locate the iso location if the default mounting
location fails

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[TheRealFalcon]

Thanks @anhvoms , your changes here look good. I left a few nits inline, but nothing major.

There's a conflict now with master. Can you rebase the PR and then we should be able to get this merged soon.

[github-actions]

Hello! Thank you for this proposed change to cloud-init. This pull request is now marked as stale as it has not seen any activity in 14 days. If no activity occurs within the next 7 days, this pull request will automatically close.

If you are waiting for code review and you are seeing this message, apologies! Please reply, tagging mitechie, and he will ensure that someone takes a look soon.

(If the pull request is closed and you would like to continue working on it, please do tag mitechie to reopen it.)

[github-actions]

Hello! Thank you for this proposed change to cloud-init. This pull request is now marked as stale as it has not seen any activity in 14 days. If no activity occurs within the next 7 days, this pull request will automatically close.

If you are waiting for code review and you are seeing this message, apologies! Please reply, tagging mitechie, and he will ensure that someone takes a look soon.

(If the pull request is closed and you would like to continue working on it, please do tag mitechie to reopen it.)