Reconfigure GitLab docker image to be able to use non-production EE_LICENSE

What does this MR do?

A bit of background - We'd like to switch to use ee_license (activation code in the future) that's generated on staging, so that it doesn't pollute production database, and we have more flexibilities with writing more new tests. Ref: Reconfigure docker image to use non-production ... (#657 - closed) So this MR:

1. Address Follow-up from "Add cloud activation scenario" (#659 - closed)

   • This MR allows tests to configure gitlab_rails['env'] in gitlab.rb file from different entries, and combine them together to write to the file

2. Add a test_license_mode variable to enable GitLab install to be able to use license/activation code generated on staging environment.

3. Remove Test::Integration::CloudActivation test scenario

   • As we now config GitLab via test_license_mode variable, we don't need special config for cloud_activation suite.

Next Step

Once the MR is merged, we need to follow steps below

1. Update CI variables in gitlab-qa project
   • Update TEST_LICENSE_MODE to true
   • Update EE_LICENSE to a staging license (value available in QA vault)
2. Update CI variables in gitlab-qa-mirror project
   • Update TEST_LICENSE_MODE to true
   • Update EE_LICENSE to a staging license (value available in QA vault)

Testing pipeline result:

Pipeline result with new EE_LICENSE value and TEST_LICENSE_MODE set to true - https://gitlab.com/gitlab-org/gitlab-qa/-/pipelines/452862392

• Failures due to !829 (comment 817181628)