Prefer service object over before_save for Ci::Runner#tick_runner_queue

A follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8084#note_21675261

Some quotes from Slack conversation with @ayufan

I’m not sure, we update that only for two conditions:

1. after runner settings are changed,
2. when a build can be picked by runner

right now we have update on runner in a few places btw. change the in runnerscontroller: update_attributes to be update and api/runners.rb so in at least 3 places

/cc @bkc