As has been noted above, Apple does not provide a configuration option for this. The lack of some third-party utility to do this leads me to believe that the APIs simply do not expose this functionality, though this is pure speculation on my part.
In my case, it isn't that the messages are too obtrusive. It is that they are too un-obtrusive in multi-monitor setups or with large, hi-res monitors (I have both).
I will get so focused on my work on a different monitor, or even the same monitor on the left hand side of the screen, that I totally miss the message window even if I have configured it to sit there until clicked. With the audio muted on my computer (as it usually is in my team's workspace), I won't notice it until something else breaks my focus. This could take hours. That totally defeats the purpose of some of the apps that use the notification center.
I need those alerts to be in my face. By that I mean on the monitor I'm working in, positioned near where my cursor is, refusing to go away until clicked, on top of everything, but not focused until I click it.
I have just spent several hours trying to come up with a workaround using Hammerspoon (http://www.hammerspoon.org/), but have yet to be successful. I will update this post if that changes.