Control incremental rollouts in the UI

Problem to Solve

Incremental rollouts driven by variables (https://gitlab.com/gitlab-org/gitlab-ee/issues/5415) are a very powerful tool, but you still need to change a secret variable in settings and to redeploy the application. We should provide a way in the UI so you can increase the number and make your incremental rollout progress up to 100%.

At the moment, going and setting project/secret variables and re-running the deployment for our Kubernetes canary deployments is not a very obvious solution and feels a little clunky. This will make the experience more directly related to the deployment operations.

Description

In CI/CD > Environments, provide a way to specify the new percentage of the incremental rollout, in a consistent way with the scaling option.

We should also add some way to explain to users how to use the feature and how to check if the deployment process is supported or not.

Design

(TODO: Where is the documentation links to?)

Tooltips

It has the short help text in a tooltip to explain the terms of Instances and Incremental rollouts.

(Note: the copy is not ready yet.)

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