Installed dashboard throw content package don't show field in the row selection

## Expected Behavior

After install a dashboard, should be allowed to edit it completely.

## Current Behavior

There is a bug that is not listing the fields in the aggregation:

## Possible Solution

## Steps to Reproduce (for bugs)

1. Create and download a content package with a dashboard with fields that doesn't exist in the other environment
2. Upload and install the content package
3. Try to insert new fields to any aggregation of the dashboard.

Not sure if this will reproduce, didn't test, but was my theory that the reason it crashed is because there was one field that was just existing in the environment that I created the dashboard.

## Context

Probably I will have to create the dashboard from the beggining. Costing time.

## Your Environment

* Graylog Version: 3.3.0+4ea5649
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: