Allow users to specify environment tier via .gitlab-ci.yml

What does this MR do?

In !55471 (merged), we've added the tier column to the environments table. This column is to specify the Deployment environment, which is one of production, staging, testing, development, or other.

In this MR, we allow users to specify the tier via .gitlab-ci.yml. We add a new key named environment:deployment_tier: in the .gitlab-ci.yml syntax:

For example, you can specify that customer-portal environment is production tier.

deploy-to-production:
  script: echo 'Deploy to production'
  environment:
    name: customer-portal
    deployment_tier: production 

In general, the tier is automatically set based on the environment:name:, but sometimes user puts an application codename (e.g. customer-portal) to an environment and an unexpected tier could be set. environment:deployment_tier is a complement in such case.

Related #300741 (closed)

Screenshots (strongly suggested)

Specify the environment tier in .gitlab-ci.yml

Deployment job passed

The tier is correctly reflected on the row

[2] pry(main)> project.environments.find_by_name('customer-portal').tier
  Environment Load (0.4ms)  SELECT "environments".* FROM "environments" WHERE "environments"."project_id" = $1 AND "environments"."name" = $2 LIMIT $3 /*application:console,line:/devkitkat/services/rails/cache/ruby/2.7.0/gems/activerecord-6.0.3.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:98:in `exec_query'*/  [["project_id", 35], ["name", "customer-portal"], ["LIMIT", 1]]
=> "production"

Specify the unknown tier in .gitlab-ci.yml

Show a proper error

Does this MR meet the acceptance criteria?

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