Update.php should automatically apply any new configuration defaults

Currently, anytime a module adds a new configuration default, the module maintainer needs to write a hook_update_N() to merge the new default into the active set. This violates DRY by putting the default in two different places, and should not be necessary for all the same reasons hook_install() doesn't need to call schema_install().

Backdrop's update process can detect changes to the default sets and apply the changes automatically.

PR: https://github.com/backdrop/backdrop/pull/1352