When creating a Chef training node derive the storageAccount used for the Chef Server

[russellseymour]

Split the Chef Server URL so that the name of the server can be retrieved. This can then be used to work out the name of the storageAccount that the machines should use for their disks.

This will prevent the need for the storageAccount to be know by the operator and stop multiple storageAccount being created.

For the training this is fine as the dnsLabelPrefix is set to the same name as the vm, however it would not be reliable in other environments where this is the case.