[Elastic Agent] Only allow installing APM Server once per policy

[simitt]

Right now the APM integration can be added multiple times to a policy, which leads to an error when applying the policy to an Elastic Agent that tries to start two APM Server.

There is no use case coming to my mind where it would make sense to have multiple APM Servers running as subprocess of one Elastic Agent.

[ruflin]

@simitt Agree. We have for the endpoint integration that it can only be configured once per policy. We should do the same here. @jonathan-buttner Where did we set this?

[axw]

I believe I have stumbled across the solution, which is to set multiple: false in the package manifest.

[jalvz]

Tested trough the UI, works as expected