Implement `delete environment`

[bijancn]

Currently gitlabctl can only get environments. I already noticed that sometimes this shows you environments that are already stopped and you don't care about anymore. There is no way to delete it from the UI but there is a possibility from the REST API https://docs.gitlab.com/ee/api/environments.html#delete-an-environment

This might need an extension of the gitlab rust base library (I currently maintain a fork here https://gitlab.com/bijan.chokoufe/rust-gitlab/commits/master and still have to figure out how to merge it to the original gitlab repo).