Limit the number of triggered builders for non-approved external PR's

To save up Buildbot resources since the queues tend to easily go over 2000 in just a couple of days of intense activity on PR's, we want to:

• have a meaningful set of triggered builders on fresh, external PR's so that a preliminary review will be conducted with a reasonable amount of build / test information.
• when the preliminary and final review are done, enable a full Buildbot run.

The mechanism could be implemented like this:

• if the PR has the label "External Contribution" and is in a non-approved state, then run the subset A of builders and for at least, one protected builder shouldn't be in that list so that the PR is un-pushable until approved.
• when at least one approval is given on the PR, it is considered in a good enough shape and, see below:
• after it is approved, a force-push can be done to trigger Buildbot again and run all the builders, or simply, from the Buildbot interface, re-run the tarball for the most recent push, it will evaluate again the state of the PR and trigger all the builders.

The subset A of builders needs to be defined. Happy to hear your feedback gkodinov, serg, danblack