Optimize kernel neighbour table for large scale environments

Changing the default values of neighbor table (also known as ARP table)
in the kernel to avoid neighbour table overflow and thus fix
communication errors between overcloud nodes.
default kernel values support L2 network up to 1024 hosts (/22).
The patch will allow up to 4096 hosts (/20).

Conflicts:
	puppet/services/kernel.yaml

Change-Id: I5fabc766dd75a38cd3d835deee7e168f04dd30ce
Closes-Bug: #1690087
(cherry picked from commit 1651a1805a)

puppet/services/kernel.yaml

@@ -26,6 +26,28 @@ parameters:
     default: 1024
     description: Configures sysctl fs.inotify.max_user_instances key
     type: number
+  NeighbourGcThreshold1:
+    default: 1024
+    description: Configures sysctl net.ipv4.neigh.default.gc_thresh1 value.
+                 This is the minimum number of entries to keep in the ARP
+                 cache. The garbage collector will not run if there are
+                 fewer than this number of entries in the cache.
+    type: number
+  NeighbourGcThreshold2:
+    default: 2048
+    description: Configures sysctl net.ipv4.neigh.default.gc_thresh2 value.
+                 This is the soft maximum number of entries to keep in the
+                 ARP cache. The garbage collector will  allow the number of
+                 entries to exceed this for 5 seconds before collection will
+                 be performed.
+    type: number
+  NeighbourGcThreshold3:
+    default: 4096
+    description: Configures sysctl net.ipv4.neigh.default.gc_thresh3 value.
+                 This is the hard maximum number of entries to keep in the
+                 ARP cache. The garbage collector will always run if there
+                 are more than this number of entries in the cache.
+    type: number
   ExtraKernelModules:
     default: {}
     description: Hash of extra Kernel modules to load.
@@ -76,6 +98,13 @@ outputs:
               # set inotify value for neutron/dnsmasq scale
               fs.inotify.max_user_instances:
                 value: {get_param: InotifyIntancesMax}
+              #avoid neighbour table overflow on large deployments
+              net.ipv4.neigh.default.gc_thresh1:
+                value: {get_param: NeighbourGcThreshold1}
+              net.ipv4.neigh.default.gc_thresh2:
+                value: {get_param: NeighbourGcThreshold2}
+              net.ipv4.neigh.default.gc_thresh3:
+                value: {get_param: NeighbourGcThreshold3}
             - {get_param: ExtraSysctlSettings}
 
       step_config: |