setup-disk: Allow specification of LVM size

It's not currently possible to specify the size of the LVM partition during installation. This would be a trivial change which would allow leaving free space on the specified root disk(s) after install.

In setup-disk.in in native_disk_install_lvm(), all that is necessary is to replace:

	local lvm_size=

with:

	local lvm_size=${LVM_SIZE}