Add disk size

[st02tkh]

Add disk_size parameter to specify desired VM disk size (in GB)

[st02tkh]

Current master doesn't pass tests in the same way as this PR.

[rgl]

This would be even greater if it had support for multiple disks, like with a disks array, that would also allow one to set the disk size, datastore, format (like govc vm.disk.create, or using the other *.disk.create variants), etc, e.g.:

disks: [
  {size: "20G", name: "sda", thick: false},
  {size: "30G", name: "sdb", thick: false},
]

[CollinLeishman]

Hey there @st02tkh, did this get bailed on?