Description of problem:
The kickstart 'partition' command insists on having a '--size n' argument
even when it is not needed; e.g. when the disk is pre-partitioned.

Version-Release number of selected component:
anaconda-21.20-1.fc21

How reproducible:
100%

Steps to Reproduce:
1. Do a kickstart installation to a pre-partitioned disk (it doesn't have
   to be formatted), with 'partition' commands like this:

    clearpart --none
    partition / --fstype ext4 --onpart sda2

Actual results:
Installation fails with "The size None is not valid".

Expected results:
Installation succeeds (as it has in, e.g. Fedora 18).

Additional info:
The failure message is emitted by pyanaconda/kickstart.py .
A workaround is to use '--size 0'.  I see no difference in the resulting
partition table or filesystem sizes; that would seem to indicate it's
just a bug in the command parser.