Sidekiq service file for systemd (CentOS, Fedora)

CentOS and Fedora uses `systemd` not usual `init.d` so it needs bit different configuration.

I added `sidekiq.service` file

first I tried:
```
ExecStart=/bin/sh -c "cd /home/gitlab/gitlab/ && bundle exec rake sidekiq:start"
User=gitlab
Group=gitlab
```
but sidekiq was crashing (don't know why, it was launched as gitlab user)

this works fine
`ExecStart=/bin/su - gitlab -c "cd /home/gitlab/gitlab/ && bundle exec rake sidekiq:start"`