RunConfig: add support for GPUs

[francescolavra]

This change adds 2 new attributes to the RunConfig JSON configuration object:

• GPUs (int): indicates how many GPUs should be attached to an instance (default: 0)
• GPUType (string): type of GPUs to be attached to an instance (cloud provider-specific)

Handling of these configuration option is implemented for the GCP cloud provider; example configuration suited for GCP:

  "RunConfig": {
    "GPUs": 1,
    "GPUType": "nvidia-tesla-t4"
  }