Add trusted certs to address #712

There is currently no easy way to add certificates to Gitlab. Adding custom certificate is required when either self-signed certificates or certificates from a CA that is not publicly trusted are used. The later is a common scenario in larger companies where an internal Root CA is used.

Gitlab ships a cacert.pem Root CA store file which is used for validating SSL/TLS certifcates when outbound connections are made; e.g. when an outgoing webhook is called (possibly to the integrated Mattermost instance).

This MR adds a recipe that automatically appends certificates placed creates the needed symlinks in #{install_dir}/embedded/ssl/certs/ for custom certificates put in the /etc/gitlab/ssl/trusted-certs directory. That should fix #712 (closed).

Feedback, improvements and testing highly welcome.