We get an NPE in added() because reverseNetwork hasn't been initialized by that point. @andreaturli is going to fix that now.

Suggestion is:

Have a method `initReverseLookupDomain` that sets three new attributes: REVERSE_LOOKUP_NETWORK, REVERSE_LOOKUP_CIDR and REVERSE_LOOKUP_DOMAIN.
Change REVERSE_LOOKUP_NETWORK  from a ConfigKey to a BasicAttributeSensorAndConfigKey.
Change the getReverseLookupDomain to just do getAttribute(REVERSE_LOOKUP_DOMAIN).

And call initReverseLookupDomain() at the beginning of preStart().