Make it easier to refresh VCR cassettes

We use VCR cassettes for tests in spec/gitlab_net_spec.rb). I encountered a few problems when trying to refresh them:

1. they point to https://dev.gitlab.org which, as far as I understand, means that I have to have .gitlab_shell_secret for https://dev.gitlab.org (which I don't have, probably won't have and also don't want to have)
2. they use a hardcoded dummy value of the said secret. This means that a person refreshing them has to locally change the tests, refresh the cassettes and change the tests back
3. they contain references to very specific data that only exists in https://dev.gitlab.org

I tried pointing the tests to my local GitLab instance. This works for example for check-ok.yml, but fails for discover-ok.yml because of point 3.

I don't know what the best solution here is. Point 3 seems to be the biggest obstacle. However, I like the fact that these tests are quite detailed (which requires depending on specific data).

Assuming that we make changes here relatively rarely, maybe we only need a better documentation that describes the process of refreshing the cassettes and this process may depend on one specific person with access rights.