support different sizes for worker disks.

[dustymabe]

Some people try to resize the provided RH CoreOS image using qemu-img resize and then are surprised when workers don't show up with larger disks (see openshift/os#318 (comment)).

Can we implement this?

cluster-api-provider-libvirt/cloud/libvirt/actuators/machine/utils/volume.go

Lines 14 to 17 in 83c10e8

	const (
	// TODO: support size in the API
	size = 17706254336
	)

[derekwaynecarr]

@dustymabe agree.