[Fleet] Handle output deletion via API and preconfiguration

[nchaulet]

Description

By allowing user to have multiple output we allow user to delete output from kibana config file or via the output API. We should have predictable behavior on we handle this deletion and the impact on existing policies.

We should never have a state where an agent policy do not have an output assigned.

I think we can have the following behaviour consistent between the API and the preconfiguration

Via API

• It should not be possible to delete a default output, (the user will have to create another default output to delete the previous default output)
• If you delete an output the existing policies using that output should move to use the default output (this will throw if the default output is logstash and an existing policy use APM)

Via preconfiguration

• If you delete an output the existing policies using that output should move to use the default output
• if you delete a preconfigured default output and do not create a new one => we should make the delete preconfigured output non managed

cc @joshdover

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)