Rename cortex cluster configure cmd to cortex cluster scale

[RobertLucian]

Cortex cluster scale command should take the following flags:

spin down a cluster

Usage:
  cortex cluster scale [flags]

Flags:
  -n, --name string    name of the cluster
  -r, --region string   aws region of the cluster
 --node-group string <required>
 --min-instances int <required>
 --max-instances int <required>
  -h, --help            help for scale

cortex cluster scale --name cortex --region us-west-2 --node-group gpu-spot --min-instances 0 --max-instances 5