Use modern datasource format

[DifferentialOrange]

Dashboard jsons specify datasource in each panel with

"datasource": "${DS_PROMETHEUS}",

and do not contain any datasource info in its target

"targets": [
  {
     "expr": "up{job=~\"$job\"} * on(instance) group_left(alias) tnt_info_uptime{job=~\"$job\"} or\non(instance) label_replace(up{job=~\"$job\"}, \"alias\", \"Not available\", \"instance\", \".*\")\n",
     "format": "table",
     "instant": true,
     "intervalFactor": 2,
     "legendFormat": "",
     "refId": "A"
  }
]

Since Granafa 9, it is transformed to

"datasource": {
  "type": "prometheus",
  "uid": "PBFA97CFB590B2093"
},

"targets": [
  {
    "datasource": {
      "type": "prometheus",
      "uid": "PBFA97CFB590B2093"
    },
    "expr": "up{job=~\"$job\"} * on(instance) group_left(alias) tnt_info_uptime{job=~\"$job\"} or\non(instance) label_replace(up{job=~\"$job\"}, \"alias\", \"Not available\", \"instance\", \".*\")\n",
    "format": "table",
    "instant": true,
    "intervalFactor": 2,
    "legendFormat": "",
    "refId": "A"
  }
],

on import.

For now, it is handled by Grafana itself, but it is possible that one day required input standard will change. grafonnet does not support 9.x datasources yet.

[DifferentialOrange]

Covered by #215