Added CustomData for provisioning VMs.

As described in http://msdn.microsoft.com/en-us/library/azure/jj157194.aspx.
It was necessary to update X_MS_VERSION to '2013-10-01' to be able to use the CustomData field.

Fixed an issue in DataVirtualHardDisk where host_caching was set by default to u''. This creates a problem since the empty string is not a valid value.