[ovirt-devel] Add "ci system-test" command

Similar to "ci build", "ci test", "ci merge" add a new command that triggers OST run. Running OST is tied now in vdsm (and engine?) to Code-Review: +2. This causes trouble and does not allow non-maintainers to use the convenient OST infrastructure. Expected flow: 1. User add a comment with "ci system-test" 2. OST flow building and running OST triggered