Allow triggering downstream check where commit or relation chain is cherrypicked

The current downstream check uses a checkout of the change to inject the sha1 to qt5 dependencies. This easily leads to it failing due to unrelated changes in parent changes or that a incomplete dependency update round has ran, which has left the qtbase incompatible with qt5. 

An option to cherry-pick the change/relation chain would allow testing only the changes from the user and excluding unrelated changes.

Ability to schedule/trigger an check for some topic in gerrit would also be beneficial. This would allow user to test multiple changes in different modules at once or create fixes for downstream modules and schedule a check to verify them.