Added service API endpoint to retrieve service parameters
requested to merge github/fork/bpetheo/bpetheo_add_service_api_endpoint_to_retrive_service_parameters into master
Created by: bpetheo
Added service API endpoint to retrive service parameters so automated scripts can get those without any extra configuration.
Related feature request: http://feedback.gitlab.com/forums/176466-general/suggestions/9590679-api-get-slack-parameters Previous pull request: https://github.com/gitlabhq/gitlabhq/pull/9602