For large scale deployment, the dvr router will be installed to
the scheduled DHCP host. This will definitely increase the l3
agent service pressure, especially in large number of concurrent
updates, creation, or agent restart.
This patch adds a config ``host_dvr_for_dhcp`` for the DHCP port
device_owner filter during DVR host query. Then if we set
``host_dvr_for_dhcp = False``, L3-agent will not host the DVR router
namespace in its connected networks' DHCP agent hosts.
Closes-Bug: #1609217
Change-Id: I53e20be9b306bf9d3b34ec6a31e3afabd5a0fd6f
8f057fb49a