Properly validate instance quota on `cortex cluster configure`

[RobertLucian]

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Description

Assuming a new node group is to be added and another one is to be removed in one go with cortex cluster configure, take into consideration the temporary increase of the vCPU cores when validating the instance quota. Currently, this isn't happening and can lead to a situation where the cortex cluster configure command will fail (due to insufficient quota)

Context

The new node group is first added and then is the other node group removed.