Explicitely set networkaddress.cache.ttl to a reasonable value by default

[lmineiro]

To avoid the many different ways/hacks this is being set I propose the bundled /etc/java-8-openjdk/security/java.security explicitly sets networkaddress.cache.ttl.

The most frequent value I've observed is 60 seconds but this is also a common DNS record TTL. 30 seconds could also be reasonable.

[hjacobs]

@lmineiro the default setting should be fine (<=30s), at least what I checked last time. What is the trigger/purpose of opening this issue now?