Don't set auto reviewers if they’re already set manually

[willinDrive]

We got a bit confusing thing. When we setting needed reviewers manually (in draft PR mode), auto-request sets number_of_reviewers reviewers anyway.

So could we make some flag in yaml that will prevent adding extra reviewers. If we set 2 reviewers manually and number_of_reviewers=3 auto-request will set 1 more reviewer. That flag could be named count_manual_reviewers