Fix AKS count validation message

[mantis-toboggan-md]

Summary

Fixes #11842 - see comments in the backport issue

Occurred changes and/or fixed issues

This PR updates AKS node pool count validation. Based off what I see in the Azure portal, AKS pools may have a count of 0 if the pool mode is 'User' - I wish I understood where I got the idea that this was actually dependent on the enableAutoScaling property last week but life is full of mysteries I guess.

Technical notes summary

I moved the validator function into a utility to make it easier to add some unit tests.

Areas or cases that should be tested

Verify that node pool count is validated to be >=1 in system pools and >=0 in user pools (ie when count is invalid an error message is shown and create button greyed out)

Screenshot/Video

Screen.Recording.2024-09-12.at.1.28.35.PM.mov

Checklist

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[eva-vashkevich]

lgtm