Reload config file without restarting the pod

[syd20]

To build upon feature request in #484, it would be helpful to have cloudprober read changes to the config file and update without the need to restart the pod. Currently I have a dynamic setup on EKS where I have multiple probes defined and changes to one requires me to regenerate the config file and then also restart the pod for the changes to take effect.

The option of using targets is not beneficial in this case but the underlying logic behind the targets file is what I would like to see for config files. With the option of reading from S3 scheduled for some upcoming release, it would be good to have a way where cloudprober reads the updated config and reloads (without a restart). Filebeat does something similar

[manugarg]

Copying from what I said on #484...

Live reload has come up a few times before. I've been reluctant of adding it because clearing a probe's (and other) state cleanly on its removal was not very well implemented. We've certainly improved in that area quite a bit and can look at it again.