Make grenade jobs voting again

This reverts the changes made to the zuul config in [0]. Since the
backports to the stable branches have been merged, the jobs can be
voting again.

[0] I5d8aa0e58e0409c54451b51de5eb70ba9a68d849

Change-Id: I3f5972e05faea8f11d7e87f3f8b05e4979e6c328
1 file changed