Periodic update of DNS entries.

The new DNS mechanism to resolve instance names on private network with
multi hosted mode doesn't scale.
This patch add the possibility to update the DNS entries periodically on
each network host in addition to fanout cast each time the DNS entries
need to be updated.

DocImpact: new config options
Fixes LP bug #1086026

Change-Id: I59094a29c9f5010f5e55a6f11160263337b058b2
Merge "Periodic update of DNS entries."