Decrease default TTL and neg-cache TTL to 60s (from 7 days)

Default in forge module dns is 604800 (7 days) for both. This does
not make sense for an internal DNS (especially not in a cloud setup).
Do no use hard coded values for SOA serial

A hard coded value prohibits slave DNS servers to do zone transfers.
Removing the serial here means we will use the defaults from
forge module dns (which is the current unix timestamp).

Regressions:
- Each puppet run on the DNS will do changes to the managed resources
- Each puppet run on the DNS results in a zone transfer
- Each puppet run on the DNS will at least reload the nameserver