Allow automated activation

What does this MR do and why?

Part of customers-gitlab-com#4646 (closed)

Allow activation to be automated during installation.

This is behind the feature flag therefore I've not included documentation update or changelog.

How to set up and validate locally

1. Start CDot
2. Enable feature flag, if you aren't using general dev environment name customers-development.
3. Create a cloud license subscription on CDot, and obtain its activation code
4. On SM instance, run bundle exec rake gitlab:license:load GITLAB_ACTIVATION_CODE=foobar

The result should be Activation successful. Check by going to Rails console and see License.last is the newly created license file.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.