nrpe relations must use nrpe-specific endpoints if available

Although we alert if a required relation to nrpe is missing, we do not check the relation type.

For charms that expose a nrpe-external-master or local-monitors endpoint, we should alert if a relation toward nrpe is present, but only uses the juju-info endpoint.

For charms that do not expose such endpoints, a relation using juju-info should be deemed sufficient.