Fix for CPU stats collection

The Cpu usage samples will just get bigger and the "@cpu_samples == 2" will never be true. Shifing the array will keep it with 2 elements.

Btw, the newest vmc is expecting to get the Cpu usage as a percentage: https://github.com/cloudfoundry/vmc/blob/0ab038aa4a3ea1609456e2d5a2dce9bf97518ade/lib/vmc/cli/app/stats.rb#L39
Where should the fix be applied, in DEA or VMC?