Increase default for NovaReservedHostMemory to 4096

Idle compute nodes are found to already consume ~1.5GB of memory, so
2GB is a bit tight.  Increasing to 4GB to be on the safe side.  Also
see https://bugzilla.redhat.com/show_bug.cgi?id=1341178

Change-Id: Ic95984b62a748593992446271b197439fa12b376

puppet/services/nova-compute.yaml

@@ -72,7 +72,7 @@ parameters:
     description: >
       Reserved RAM for host processes.
     type: number
-    default: 2048
+    default: 4096
     constraints:
       - range: { min: 512 }
   MonitoringSubscriptionNovaCompute:

releasenotes/notes/increase-nova-reserved-host-memory-80434e8484a29680.yaml

@@ -0,0 +1,4 @@
+---
+other:
+  - Increased the default of NovaReservedHostMemory for
+    Compute nodes to 4096 MB.