Listen for Docker events for hot reload

[MrMohebi]

Replaces polling with event listening for updating labels.

[MrMohebi]

@taraspos Your review has been simplified and implemented—thanks!

[taraspos]

Thanks @MrMohebi!
I've also added some additional filtering for events!

Before merging this, I would like to add some integration test, to make sure that configurations are being refreshed properly, etc.

If you would like to try to add one yourself, you can find example test here:

• https://github.com/mcuadros/ofelia/blob/master/integration/run_exec_test.go

If not, I will try to add one somewhere later myself.

[taraspos]

@MrMohebi sorry for the delays on this one.
One of the tests is failing for some reason, I need to come back and find what's going on in there.