Set the default mac ageing time to 600 seconds (#2365)

* Set the default mac ageing time to 300 seconds

The current mac ageing was disabled, this could lead the mac address
table to increase over time and lead to resource and performance issues.

Signed-off-by: Zhenggen Xu <zxu@linkedin.com>

* Update the default HW ageing timer to be 600 seconds.

This is to be on the safer side where ARP update interval
is 300 seconds and SONiC does not flood when ARP is aged out.

Signed-off-by: Zhenggen Xu <zxu@linkedin.com>

dockers/docker-orchagent/switch.json.j2

@@ -12,7 +12,8 @@
     {
         "SWITCH_TABLE:switch": {
             "ecmp_hash_seed": "{{ hash_seed }}",
-            "lag_hash_seed": "{{ hash_seed }}"
+            "lag_hash_seed": "{{ hash_seed }}",
+            "fdb_aging_time": "600"
         },
         "OP": "SET"
     }