Replace objectName with nameSelector.matchNames

[dmgtn]

Reasons:

1. We already have matchNames in namespaceSelector
2. matchNames will allow matching for more than one object at a time

To my eye, objectName looks extremely ugly and non-Kubernetes-API-common, so we better use matchNames from prometheus-operator – at least we will be not the only ones using this keyword. And at least we will use the same approach both for namespace matching and for the object matching.

[diafour]

1. The selector field is nameSelector.matchNames as of Hooks configuration API refactoring #37
2. Document this feature in HOOKS.md