Add Praefect to the gitlab.rb in omnibus

Omnibus generates the configuration files for all the services that GitLab needs to run. These settings are set in a Ruby file, called gitlab.rb. This file should get a section for Praefect to set the variables which than in turn generate a config.toml.

The second part of this issue would allow for praefect to be run as a service like all other, leveraging gitlabctl to start, stop, etc, etc. This part of the issue probably needs guidence, but could probably be copied from the Gitaly daemon, or the GitLab pages one.

Including the binaries was done through: #1498 (closed)