fixes a regression introduced in #14269 that refactored
the notification registration logic, all the amqp targets
however online will not be available for use anymore.
fixes#14451
When setting a config of a particular sub-system, validate the existing
config and notification targets of only that sub-system, so that
existing errors related to one sub-system (e.g. notification target
offline) do not result in errors for other sub-systems.
This is to ensure that there are no projects
that try to import `minio/minio/pkg` into
their own repo. Any such common packages should
go to `https://github.com/minio/pkg`
Harshavardhana
Klaus Post
Shireesh Anjal
Poorna