Differentiate between OS Disk and Data Disks in Azure VMs

[tloisel1]

The current implementation of Azure VM in gyro only has a single disk field that seems to control the OS Disk.

We need the ability to add Data Disks as well. The fields for these resources should be clearly differentiated as they serve very different purposes.

There also needs to be a mechanism to differentiate between Disks that are implicitly created via Azure API's and ones defined in Gyro and attached to the VM. It is my view that the implicitly created ones should be destroyed when the VM is destroyed. This expands on the issue defined in #53.