Provide a gitlab-ctl command for using Mattermost CLI with settings provided as environment variables

Since 11.0, we pass configuration to Mattermost as env variables. This means, users can't simply initiate mattermost binary with just the config file and expect the CLI to work fine, as it will be missing many settings. The workaround is being documented in !3644 (merged), but I think it would be easier for our users if we provide a gitlab-ctl mattermost-cli command which essentially does everything needed.