Need to allow disabling of maas-proxy service checks in nrpe

For newer versions of maas. The maas-proxy no longer runs by default if proxy is set to external or disabled.
This seems like the correct behavior for maas but we will not need to avoid alerting if the maas-proxy service is down intentionally.

In the past the proxy would run even if it was set to external or disabled in maas.

From what I can tell maas does not disable the service or leave any other hints on the filesystem so we will most likely need to add a disable-maas-proxy-checks to the charm configuration.