plugins/discovery: ensure discovery doesn't erase its own config

When the discovery plugin receives a a discovery bundle which omits the
discovery configuration, it deletes its own configuration from the
manager. In turn this means that `GET /v1/config` doesn't show the
configuration of the discovery plugin.

This change ensures that the plugin will never overwrite the discovery
configuration on the manager.

Signed-off-by: Benjamin Nørgaard <mail@blacksails.dev>