fix aks region being overwritten when editing unprovisioned aks clusters

[mantis-toboggan-md]

Summary

Fixes #11841

This bug is specific to AKS clusters that have been created in Rancher, but not yet created in AKS. To repro, you can use the default AKS configuration options (except region) but you need to edit the cluster immediately after creating it to see the bug. This PR tweaks some login in the location-fetching method that failed to account for this inbetweeny edit mode.

Areas or cases that should be tested

1. Create a new AKS cluster - after entering in credentials, regions should be loaded and east us selected by default.
2. Change the region to something else
3. Set other required fields and save
4. immediately edit the cluster. The region dropdown should show whatever region you picked in step 2. If you open the dropdown, you should see a list of regions to select.

Checklist

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