stub and slave zones need to be overwritten with data from their master server
in a reasonable timeframe. Drop all of the intervals in the db.empty SOA down
to sub-minute values to speed convergence.
This adds a 'dnssec' parameter to the bind::zone define which causes the module
to generate keys and sign the zone. Some caveats and breaking changes:
1) Existing non-signed zones will have to be manually moved and signed
2) Signed zones are treated as dynamic
