Temporary fix for dashboards under 7.0

[ruflin]

As some of libbeat fields were renamed in 7.0, currently a few dashboards break that depend on this fields. Adding an alias for the old fields is to temporarly fix this. As soon as the ECS migration is done, fields in the dashboard have to be renamed. Hopefully this can happen in an automated way.

[ruflin]

@tsg I tested this locally a bit and it seems to fix most dashboard issues. There is still something odd when I go to "System Host View" directly which I think is related to host.name in some way but when I go from the overview and click on the host it all works. I see this as an easy temporary fix for now

[ruflin]

This PR initially broke our tests. The reasons is that fields.ecs.yml is not taken into account when generating the fields.yml in libbeat which we use for the testing. It does not have an affect on the final fields.yml for each Beat as there the ecs fields are included. We should adjust our tests to not depend on the fields.yml in libbeat (later).

[tsg]

👍 I approve of this.

[ruflin]

@tsg I had to rebase again because of a conflict and want to make sure builds still go green. So if I don't get to merge it today, could you merge for me?