Provider does not allow dynamic scaling

Hello there,

it seems like the provider does not allow dynamic scaling of instances.
Currently the provider calls `updateVirtualMachine` everytime something has changed on a `cloudstack_instance` resource. This updates the compute instance in CloudStack Management but does not change the actual VM.
To actually change the VM `scaleVirtualMachine` must be called.

The provider should check if `details.cpuNumber`, `details.cpuSpeed` or `details.memory` has changed and call `scaleVirtualMachine` prior to `updateVirtualMachine` or skip `updateVirtualMachine` at all when nothing else changed.

Thank you for looking into this.