Improve config handling in server notifier "k8sbundle"

[edwbuck]

The server "notifier" plugin "k8sbundle" has a combined set defaults / check configs / set items / create kube clients / etc.

This convolutes the configuration validation with logic that impacts the k8sbundle plugin, making configuration validation embedded within logic about how to use the configuration.

We need to separate out this logic such that we can have the buildConfig perform the validation and any default setting without immediately triggering its use. Then after all elements of the configuration is valid, we can create the kube clients / set non-configuration items / etc.

Discovered during work on #5303 and not included in that effort to avoid scope expansion.