Add support for gitaly tls

Part of gitaly#1324 (closed)

Adds an environment variable that allows for enabling gitaly tls. In case it is provided and its value is "true" we attempt to use the system certificate pool for that communication.

Providing a custom certificate chain in runtime like it is done in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22602 is not supported mainly because no specific config is given to gitaly via config files which makes it quite complicated to do it like it is usually done right now (passing args or environment variables since we still don't have args parsing here)

This means that the CA needs to be installed locally for it to be used