Notify dnsmasq service after configuration changes

`ci-master-1` could not recognise a new CI slave that I brought up until
I restarted `dnsmasq`. We should notify the service on any configuration
changes.
Fix indentation
Notify primary class, not service class

It's better to notify the main `dnsmasq` class, rather than a specific
class underneath it as we should not have to care about the class
internals of external modules.
Merge pull request #249 from alphagov/82935274-notify_dnsmasq

[#82935274] Notify dnsmasq service after configuration changes