Updated Elixir_AAI custom service proxy tutorial.

Try to fulfill the following points before the Pull Request is merged:

[X] Please create your prs against the dev branch
[ ] Please WAIT for the tests to finish.
[ ] All tests have been successfully completed.
[X] When adding a new documentation or tutorial, make sure that the markdown is linked in the config.yml and config_dev.yml

deNBI / cloud-user-docs