Add ability to exclude Gerrit changes with no reviewers invited

[danrodrig]

It's common to have for some time Gerrit changes without reviewers invited. For example, the CI failed and the change needs a new PS to be fixed. In these cases, these changes shouldn't be counted as reviews. This change adds ability to skip changes with no reviewers and also allows to exclude users from the reviewers in the change. This can be used to not count as reviewers bot accounts used, for example, by the CI.

[amarza-rh]

Hi, please rebase from master and fix merge conflicts.

[pbortlov]

@danrodrig please ^

[danrodrig]

@amarza-rh @pbortlov rebased

[amarza-rh]

LGTM now