reduce default lease time to 5 minutes
reduce default lease time to 5 minutes

Devstack used in development is about things coming and going
quickly. The long dhcp leases mean that we might miss a release, and
keep a stale lease around for way too long. See if this helps.

Change-Id: I9a58a4e64777f56ad7ec66242a319f985469469e

noironetworks/devstack,dtroyer/devstack,Mahito/devstack,richliu/devstack,noironetworks/devstack,bigswitch/devstack,dtroyer/devstack,dirkmueller/devstack,group-policy/devstack,bswartz/devstack,openstack-dev/devstack,nawawi/openstack,samgoon/devstack,openstack-dev/devstack,samgoon/devstack,bswartz/devstack,dirkmueller/devstack,group-policy/devstack,nawawi/openstack,Mahito/devstack,bigswitch/devstack,richliu/devstack