One of the biggest complaints we get from users not being able to finish
their rollouts is that their incumbents are unhealthy, and one of the
biggest causes for that is when Deployments disappear[1] from the target
clusters. This happens because the installation controller does not even
try to replace objects after its first pass is completed, and
CanOverride is set to false.

Now, the installation controller will do a full run of the installer on
every sync, reinstalling any objects that are gone, whatever their kind.
Additionally, we'll start to listen to Service and Deployment events, so
that we can sync the InstallationTarget whenever someone messes with
these objects in the target clusters. We do this only for Deployments
and Services because they're the only ones that can cause shipper to
hang, as it depends on them in the capacity and traffic controller,
respectively.

[1] Users swear they haven't deleted them. We have yet to find another
cause for this mistery.