Limit number of environments per project/pipeline

From https://gitlab.com/gitlab-org/configure/general/issues/4

Limiting number of deployment jobs (or unique environments) defined in .gitlab-ci.yml. Even though we have limits on runner taking the jobs we may have a problem during the preparing stage which is run in a GitLab worker as this preparing stage will be still trying to create a K8s namespace and service account for every environment.

Need to also consider that generic many dynamic review app environments is common so limit should probably be high overall. We could have a much lower limit on the number of environments per pipeline since it is not really usual to want to deploy to many environments in a single pipeline run.

We also want to make this configurable so SRE or on-prem customers can tweak it as necessary.

Definition of Done

• limit the number of deployments per pipeline to 500 by default
• make this limit configurable
• document this configuration