Not set fip to ERROR if it exists on device

The centralized floating IP can exist on the router device
due to some reasons like: uncleaned fip addr, and especially
multiple IP addr adding action: HA router _add_vip() and
Edge router add_ip_address().
This patch catches the IpAddressAlreadyExists error if fip
was already set on the device, and still process next step.

Change-Id: I324f6b96baa0520a0f7ef62a83d81864d7b27999
Closes-Bug: #1811213
(cherry picked from commit 4d45699f15)

neutron/agent/l3/dvr_edge_router.py

@@ -314,10 +314,11 @@ class DvrEdgeRouter(dvr_local_router.DvrLocalRouter):
             return
         interface_name = self.get_snat_external_device_interface_name(
             self.get_ex_gw_port())
-        device = ip_lib.IPDevice(
-            interface_name, namespace=self.snat_namespace.name)
         try:
-            device.addr.add(fip_cidr)
+            ip_lib.add_ip_address(fip_cidr, interface_name,
+                                  namespace=self.snat_namespace.name)
+        except ip_lib.IpAddressAlreadyExists:
+            pass
         except RuntimeError:
             LOG.warning("Unable to configure IP address for centralized "
                         "floating IP: %s", fip['id'])

neutron/agent/linux/ip_lib.py

@@ -972,6 +972,7 @@ def get_device_mtu(device_name, namespace=None):
 
 NetworkNamespaceNotFound = privileged.NetworkNamespaceNotFound
 NetworkInterfaceNotFound = privileged.NetworkInterfaceNotFound
+IpAddressAlreadyExists = privileged.IpAddressAlreadyExists
 
 
 def add_ip_address(cidr, device, namespace=None, scope='global',