[AKS] code bug in _handle_merge which impacts aks_get_credentials

The below code deletes elements in `existing[key]` while looping through it, this would cause a skip of the element right after the one that is deleted (as the iterator will advance when deleting happens) and will cause bugs if two consecutive items need to be deleted.
https://github.com/Azure/azure-cli/blob/3226fae922e423cdc46bf3bf6f14ac9e57a66ea0/src/azure-cli/azure/cli/command_modules/acs/custom.py#L1108-L1114

Run the below python code to show the bug:
```
list=[1,2,3,4]
for n in list:
list.remove(n)
print(list) # the result is [2,4] instead of []
```