add support for the gitlab web terminal

Requirements: https://docs.gitlab.com/ee/user/project/clusters/index.html#web-terminals

Extend helm command with:

    helm upgrade --install \
[...]
      --set gitlab.app="$CI_PROJECT_PATH_SLUG" \
      --set gitlab.env="$CI_ENVIRONMENT_SLUG" \

(pulled out of /usr/local/bin/auto-deploy inside image registry.gitlab.com/gitlab-org/cluster-integration/auto-deploy-image:v0.1.0 )