Structured logging for gitlab-runner

For centralized logging of the runners we have what is logged to syslog which is unstructured and mixed in with other system logs. To improve this, gitlab-runner should have a dedicated json structured logging. This will make it much easier for us to setup centralized logging using the existing logging infrastructure.

Possible related issues:

• #2406 (closed): Add the ability to specify Logrus formatting in config.toml
• #2407 (closed): Add the ability to configure the the syslog facility