Default dhcp lease time of 120s is too short

As part of reasonable-defaults, increase the default DHCP
lease time to a more sane value of 24 hours.  This change
also aligns Nova with the current default DHCP lease time
in Neutron.

DocImpact: This changes the default dhcp_lease_time from
120 seconds to 86400 seconds (24 hours).  If the Icehouse
or previous release behavior is desired, set the config
dhcp_lease_time to 120.

Change-Id: Ia666ff31f299c57d46605705e9ebea21630e0722
Closes-Bug: #1317654
Merge "Default dhcp lease time of 120s is too short"