DVR: Ignore DHCP port during DVR host query

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

neutron/common/utils.py

@@ -170,16 +170,18 @@ class exception_logger(object):
         return call
 
 
-def get_other_dvr_serviced_device_owners():
+def get_other_dvr_serviced_device_owners(host_dvr_for_dhcp=True):
     """Return device_owner names for ports that should be serviced by DVR
 
     This doesn't return DEVICE_OWNER_COMPUTE_PREFIX since it is a
     prefix, not a complete device_owner name, so should be handled
     separately (see is_dvr_serviced() below)
     """
-    return [n_const.DEVICE_OWNER_LOADBALANCER,
-            n_const.DEVICE_OWNER_LOADBALANCERV2,
-            n_const.DEVICE_OWNER_DHCP]
+    device_owners = [n_const.DEVICE_OWNER_LOADBALANCER,
+                     n_const.DEVICE_OWNER_LOADBALANCERV2]
+    if host_dvr_for_dhcp:
+        device_owners.append(n_const.DEVICE_OWNER_DHCP)
+    return device_owners
 
 
 def get_dvr_allowed_address_pair_device_owners():