Do not change service names as we do not own them (bnc#870803)

Resolves such an issue:
```ruby
services = {"sshd"=>"start"}
services.each do |name, action|
  service = SystemdService.find(name) # RuntimeError: can't modify frozen String
end
```

A typical use case might be a key in a hash (thanks to @mchf for hint)