Add Parameter for OS Disk Size

[markcowl]

Add parameter for DiskSizeInGB

[MiYanni]

@twitchax We don't have this param. Do we need it? 🔦

[markcowl]

I don't believe this has come up in any of the tutorials.

[twitchax]

I think we should do this, but the priority is low right now.

[sergey-shandar]

New-AzureRmVM & New-AzureRmVmss ?