Add server_name to instance and instances data source

[ismirlia]

Someone pointed out that the server_name was not listed as part of the instances data source. That seems like an oversight, and should make this data source easier to use.

This is what the data source output looks like now (this one element in the list):

...
{
      "health_status" = "OK"
      "license_repository_capacity" = 0
      "max_virtual_cores" = 8
      "maxmem" = 64
      "maxproc" = 8
      "memory" = 8
      "min_virtual_cores" = 1
      "minmem" = 2
      "minproc" = 1
      "networks" = tolist([
        {
          "external_ip" = ""
          "ip" = "196.168.2.48"
          "macaddress" = "fa:1f:37:65:21:21"
          "network_id" = ""
          "network_name" = "FLS Network (Do not use)"
          "type" = "fixed"
        },
        {
          "external_ip" = ""
          "ip" = "192.168.193.125"
          "macaddress" = "fa:1f:37:65:21:20"
          "network_id" = ""
          "network_name" = "FLSnetwork(Do not use)"
          "type" = "fixed"
        },
      ])
      "pin_policy" = "none"
      "placement_group_id" = "none"
      "processors" = 1
      "proctype" = "dedicated"
      "pvm_instance_id" = "dd8ea5b7-8650-4148-8736-6236601bf3af"
      "server_name" = "MattRPMBuildServer"
      "shared_processor_pool" = ""
      "shared_processor_pool_id" = ""
      "status" = "ACTIVE"
      "storage_pool" = "General-Flash-77"
      "storage_pool_affinity" = true
      "storage_type" = "tier3"
      "virtual_cores_assigned" = 1
    },
...

[yussufsh]

lgtm
cc @hkantare