* De-share the Handler struct in core API
An upcoming PR adds a handler that only applies on one of these paths.
Having fields that don't work seems bad.
This never should have been shared. Lifecycle hooks are like a "write"
while probes are more like a "read". HTTPGet and TCPSocket don't really
make sense as lifecycle hooks (but I can't take that back). When we add
gRPC, it is EXPLICITLY a health check (defined by gRPC) not an arbitrary
RPC - so a probe makes sense but a hook does not.
In the future I can also see adding lifecycle hooks that don't make
sense as probes. E.g. 'sleep' is a common lifecycle request. The only
option is `exec`, which requires having a sleep binary in your image.
* Run update scripts
In the case of multinode clusters, the http server pod and the test cluster can
spawn on different nodes, which can be problematic for poststart / prestop hooks,
as they are executed by the kubelet itself, and the cross-node lifecycle hook might
fail (according to the Kubernetes network model, it is not mandatory for kubelet to
be able to access pods on a different node).
This commit ensures that the test pod spawns on the same node as the http server pod.
