Add an option to override default gerrit http credentials

[maksonlee]

No description provided.

[aquarellian]

Thank you for the contribution! And sorry for the delay - I missed the notification about your pull request.

Regarding the change, I'd add a button "Test connection", similar to the way how it is implemented in the Gerrit Trigger.
If you have time, I'd be grateful if you can do it. Otherwise I'll do it myself, let me know.

I'll merge your change to the plugin shortly.

[maksonlee]

When job is running, a build object is passed to peform() method so you can get related gerrit information from it, but how to do that when we are configuring job?

[aquarellian]

The only way I've found is to allow user to choose a Gerrit server to be used for credentials check. Thank you for the contribution, this feature will be available in the next release