MINOR: dns: default "hold obsolete" timeout set to 0

MINOR: dns: default "hold obsolete" timeout set to 0

The "hold obsolete" timer is used to prevent HAProxy from moving a server to
an other IP or from considering the server as DOWN if the IP currently
affected to this server has not been seen for this period of time in DNS
responses.

That said, historically, HAProxy used to update servers as soon as the IP
has disappeared from the response. Current default timeout break this
historical behavior and may change HAProxy's behavior when people will
upgrade to 1.8.

This patch changes the default value to 0 to keep backward compatibility.