use read/write cache on azure masters

Change Azure Masters to use ReadWrite cache.

The write is then lazily written to the disk when the cache is flushed
periodically. Customers can additionally force a flush by issuing an
f/sync or fua command.

etcd should be ok with this change since etcd does fsyncs when needed.

https://learn.microsoft.com/en-us/azure/virtual-machines/disks-performance

Author: rphillips

data/data/azure/cluster/master/master.tf

@@ -114,7 +114,7 @@ resource "azurerm_linux_virtual_machine" "master" {
 
   os_disk {
     name                   = "${var.cluster_id}-master-${count.index}_OSDisk" # os disk name needs to match cluster-api convention
-    caching                = "ReadOnly"
+    caching                = "ReadWrite"
     storage_account_type   = var.os_volume_type
     disk_size_gb           = var.os_volume_size
     disk_encryption_set_id = var.disk_encryption_set_id