…stic#173353)

Closes elastic#173237

Removed restriction to allow using remote es output as integration data
output.

- Create a remote es output, verify that the output is allowed to be set
as default for agent integrations
- Create an agent policy with system integration and set the remote es
output as integration data output
- Enroll an agent to the agent policy
- Check the remote kibana - Discover, verify that system metrics are
coming in from the agent
- Install system package on the remote cluster to see dashboards,
mappings, etc.
- Add nginx integration to the agent policy
- Create a dummy nginx log file in `/var/tmp/nginx/access.log` and add
some dummy data to it
- Verify that the data from the nginx log file appears in the remote
kibana Discover in `logs-*` data view.
- Change the agent policy integration output back to default
- Verify that the system integration data is ingested in the main
cluster.
- Verify that the API key is invalidated in the remote cluster

<img width="1742" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/47ed4e89-e761-4f24-90c3-bf3a49a6b4f1">

<img width="937" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/2e41f37e-a4ef-4f18-aed0-d4160efe306a">

<img width="2162" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/bf5d900b-f3bd-493e-b61a-4554224a97fc">

<img width="2150" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/31e4f32e-8751-4b02-855f-dc7fbd5e64a9">

System dashboard on remote cluster populated:
<img width="2157" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/dfb10791-ab15-4058-9170-7cad51935493">

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios