Change dhcp_lease_duration to Havana default of 86400

In Havana - the default lease time was upped to 86400 - 1 day - this should be changed back to the correct default in the puppet module as well.
http://docs.openstack.org/havana/config-reference/content/section_networking-options-reference.html

Change-Id: Ifd75f024186a9100f8aeb78415b4b5c8482913e2