Don't raise FileNotFoundError during disabling keepalived

In case when keepalived's config is not existing already, there is no
need to raise any exception while L3 agent is trying to clean this
config.

In stable/rocky branch, where we are using Python 2.7 it requires to
catch EnvironmentError and check errno instead of catching
FileNotFoundError which isn't available in Python 2.7.

Change-Id: I9ec81ad0c10379294d3145c5902e8b81b65c0221
Closes-Bug: #1892866
(cherry picked from commit a08893368a)

neutron/agent/l3/ha_router.py

@@ -12,6 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import errno
 import os
 import shutil
 import signal
@@ -193,7 +194,11 @@ class HaRouter(router.RouterInfo):
             return
         self.keepalived_manager.disable()
         conf_dir = self.keepalived_manager.get_conf_dir()
+        try:
             shutil.rmtree(conf_dir)
+        except EnvironmentError as e:
+            if e.errno != errno.ENOENT:
+                raise
 
     def _get_keepalived_instance(self):
         return self.keepalived_manager.config.get_instance(self.ha_vr_id)