Document Operator environment variables

Follow-up discussion from !360 (merged).

Controller environment variables are not documented. They can be useful for power users, esp. that they are used in GitLab Operator Chart. Here is the list of the current variables:

• HELM_CHARTS: Helm Charts discovery directory, default /charts
• GITLAB_MANAGER_SERVICE_ACCOUNT: Name of the manager ServiceAccount, default gitlab-manager
• GITLAB_APP_SERVICE_ACCOUNT: Name of the application ServiceAccount, default gitlab-app
• NGINX_SERVICE_ACCOUNT: Name of the nginx-ingress ServiceAccount, default gitlab-nginx-ingress

There is also:

• GITLAB_OPERATOR_SHARED_SECRETS_JOB_TIMEOUT