Signed-off-by: Benjamin <benjamin@yunify.com>

This patch adds a few out of the box receivers that only need their
notification provider configuration filled in, instead of figuring out
all the wiring for critical alerts for example.

Signed-off-by: Benjamin <benjamin@yunify.com>

Note that, in effect, this also leads to kube-prometheus using
the latest patch-release of prometheus-operator, 0.30.1.