Add json tags to output fields (#269)

output now:
```
{
  "run_page_url": "https://adb-309687753508875.15.azuredatabricks.net/?o=309687753508875#job/1077573342009637/run/19099317",
  "task_outputs": {
    "my_notebook_task": {
      "result": "computed results from notebook."
    }
  }
}%
```

bundle/run/job_output.go

@@ -13,10 +13,10 @@ import (
 
 type JobOutput struct {
 	// URL of the job run
-	RunPageUrl string
+	RunPageUrl string `json:"run_page_url"`
 
 	// output for tasks with a non empty output
-	TaskOutputs map[string]RunOutput
+	TaskOutputs map[string]RunOutput `json:"task_outputs"`
 }
 
 // TODO: Print the output respecting the execution order (https://github.com/databricks/bricks/issues/259)

bundle/run/task_output.go

@@ -11,8 +11,8 @@ type NotebookOutput jobs.NotebookOutput
 type DbtOutput jobs.DbtOutput
 type SqlOutput jobs.SqlOutput
 type LogsOutput struct {
-	Logs          string
-	LogsTruncated bool
+	Logs          string `json:"logs"`
+	LogsTruncated bool   `json:"logs_truncated"`
 }
 
 func structToString(val interface{}) (string, error) {