config/schema: add <schema object>:validate()

The config module should ensure that a received configuration data
corresponds to a given schema before start to process it.

This commit implements a simple validation procedure.

There are possible optimization opportunities, such as generation a
validation code by a given schema.

No optimization tricks are used here, because the configuration
validation occurs at the configuration update. It is not a frequent
operation.

Part of #8725

NO_DOC=the module is for internal use from the config code
NO_CHANGELOG=see NO_DOC