vm/vmss: full support for caching mechanism of managed disks

[yugangw-msft]

We should support

• For vm/vmss create: expose caching on data disks, as it become clears that data disk needs different caching from os disk
• Support no caching
• For vm/vmss disk attach, expose caching argument as well

[tjprescott]

Currently we use the --storage-caching argument. With this change it will be unclear what this refers to without reading the help. I would recommend the following for VM/VMSS create:

• Expose --storage-caching, --os-caching for the OS cache and --data-caching for the data disk cache. This will allow either alias to work for the OS caching.
• At some point in the future, retire --storage-caching

@yugangw-msft @johanste thoughts?

[tjprescott]

@yugangw-msft when I run VM/VMSS create with "null" sent for the caching values, here is what the service defaults to:

VM: OS - ReadWrite, Data - None
VMSS: OS - None, Data - None

Previously, we had VMSS default to "ReadOnly". Should we stick with the service defaults or do we have good reason to override them? What should the default be for data disks?