adapt gitlab plugin to nornir 3.0.0

[dbarrosop]

Relates to #486 and #504

Adapt plugin to nornir 3.0.0.

For reference look at https://github.com/nornir-automation/nornir_napalm.

Don't forget to add details to the plugin repository. You can do so by adding it here

If you have any questions don't hesitate to ask.

[dbarrosop]

@wvandeun

[m1009d]

Hi guys @dbarrosop @wvandeun , any progress on this plugin?

[wvandeun]

I had started to work on this, but have not been able to finalize it due to other priorities.
Also, I personally don't have a need for the plugin anymore. It's going to be hard for me to maintain this moving forward.

What I had already done is available here:
https://github.com/wvandeun/nornir_gitlab

Feel free to use it as a starting point.