Make `lun` field optional in Azure provider.

[Hardik Dodiya]

What would you like to be added: With the support of multiple-worker disks in gardener/machine-controller-manager#397 , luns have been made required field in azuremachineclass. We might want to consider making it optional for better UX.

Refer to the comment for related discussion.

cc @dkistner

[Samarth Deyagond]

This has been handled in MCM-Provider-Azure as well. Here are the lines of code for handling the disks and even in the MachineClass ProviderSpec, here the LUN is an optional field.

@dkistner Could you please take a look and help me decide if this issue can be closed?

[Dominic Kistner]

As the lun can be specified optionally I guess it is ok to close.