Support CI_SERVER_TLS_CA_FILE for the Gitlab Merge Request Comments Reporter

[ivanbaldo]

The Gitlab Merge Request Comments Reporter has a GITLAB_CERTIFICATE_PATH, but maybe if not defined we can use the value of the standard CI_SERVER_TLS_CA_FILE instead, so users don't have to define extra variables.

[nvuillam]

@ivanbaldo I'm not aware of this file, but if you think it could work we'll be glad to accept your Pull Request :)