GCP: custom instance types improvements

[ekazachkova]

The following PR brings the following changes for GCP provider:

• Support custom extended instance types
  • -ext suffix shall be added to the instance type ending (custom-2-65536-ext)
• Support VM families for custom instance types
  • a new field family added to region#customInstanceTypes entity. Example:

    ...
    {
    "cpu": 2,
    "ram": 64,
    "family": "n2"
    },
    ...

  • allowed families: n1, n2, n2d, e2
  • Not available for GPU
  • gcp.sku.mapping preference family pattern: <family>-custom (n2-custom). Example:

    ...
    "ram_ondemand_n2-custom": {
    "prefix": "N2 Custom Instance Ram",
    "group": "RAM"
    },
    ...

  • <family>- prefix shall be added to the instance type beginning. Example: n2-custom-2-1536 or gpu-n2-custom-2-1536-k80-1