[FEATURE] Add discard option to vm disk creation

[dmeulen]

One of the great features in proxmox is the discard option on disk volumes. Enabling the discard option on disks in comnbination with an fstrim / cron job or qemu agent fstrim inside the vm would free up discarded space automagically.

An extra option discard=<ignore|on> should be added to diskOptions when setting the virtual machine options in func resourceVirtualEnvironmentVMCreateCustomDisks.

[danitso-dp]

@dmeulen I'm adding this to the v0.5.0 milestone