plugin/ready: fix Reset list of readiness plugins

[yangjunmyfm192085]

1. Why is this pull request needed and what does it do?

When ready starts, it adds all implementors of readiness to its list of plugins to check for readiness.
We need to clears out the list, on every reload (such as kubernetes).

However, the current method of clears out the list is placed in the c.OnStartup function, which causes each plug-in to be cleared out, and there will be only one plug-in in the final list.

2. Which issues (if any) are related?

3. Which documentation changes (if any) need to be made?

4. Does this introduce a backward incompatible change or deprecation?

[yangjunmyfm192085]

/cc @chrisohaver

[chrisohaver]

I'm not clear on precisely what is currently broken.

[yangjunmyfm192085]

For example, there are two plug-ins, plugin1 and plugin2, both requiring readiness checks, but according to the current implementation, only one plug's readiness check will take effect.