Need an easy/automated way to restart a CP member

[lprimak]

Currently, when a CP group loses a member (crash or a graceful exit),
there is no easy and automated way to reconstitute the CP group to it's original member count.
One needs to go to the management console, or use API to manually add the new member to the CP group.
This is error-prone, labor intensive, and incompatible with such environments as managed cloud or kubernetes.
There should be a way for the software to automatically join the CP
when there are not enough live existing members currently.

I managed to write the code below, but it's not trivial.

This is a follow-on to #24897 (comment)