azure: add support for storage_profile_data_disk for workers

[invidian]

storage_profile_data_disk allows attaching data disks into Azure Scale Set we use for workers and I think this is a good way of providing disks for OpenEBS on Azure.

There are 2 things which still needs to be addressed. Once new worker is provisioned it requires manual action of preparing a disk, which is sudo umount /mnt/resource && sudo sgdisk --zap-all /dev/sdb.

I also think, that data disks should be optional. Since storage_profile_data_disk is a block, we should wait until migration to Terraform 0.12 is done, so we can support optional blocks with dynamic syntax. Terraform reference issue: hashicorp/terraform#7034