Define a default rollout strategy

What does this MR do?

Defines a default rollout strategy at the ApplicationExperiment level.

I don't believe the RoundRobin strategy is going to be the complete one, but it's the one with the most predicable behavior right now, so let's try it and build a new one if we need to. This needs to be present for the docs at https://docs.gitlab.com/ee/development/experiment_guide/gitlab_experiment.html to be fully correct.

Does this MR meet the acceptance criteria?

Conformity

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