Increase lease time for dhcp agent

Usually failover takes a couple of minutes. 120 seconds is not enough so
during failover VMs loose IP address, breaking the connectivity.
Increasing the value to 600 allows

* Minimaze the number of DHCPDISCOVER requests during failover
* Minimaze the load on dnsmasq as it will be less frequently renewing IP
  addresses for VMs

Change-Id: I7dc57e3f79ed8ee5e83c26e0c80d6a44e0840b4a
Closes-Bug: 1371104
Merge "Increase lease time for dhcp agent"