clone: support for creating additional disks

[michaelw]

Option "--disks 32 64" adds two additional disks, 32GB and 64GB.

[snobear]

@michaelw great addition, thanks. Per the arg description:

...
        help="Additional disks in GB, e.g. 16,thin 32; "
             "adds two disks, 16GB (thin provisioned) and 32GB (default)",
        nargs="+",
...

shouldn't that say e.g. --disk 16 thin 32 ?

Also can you fix the pep8 errors per the Travis build?

[michaelw]

updated.

re the example, --disks SPEC1 SPEC2, where SPECi consists of SIZE,FLAGS... is easier to extend in the future, if needed.