Description of problem:
The following kickstart command, which worked as expected in older versions of kickstart, errors with "Partitions required for raid":

raid /boot --fstype=ext4 --level=1 --device=md0 --useexisting

Partitions should not be required when --useexisting is used.  The cause is commit 3fa478da.. which added this check at pykickstart/commands/raid.py:207:

         if len(extra) == 1:
             raise KickstartValueError, formatErrorMsg(self.lineno, msg=_("Partitions required for %s") % "raid")

That should allow for --useexisting.

Version-Release number of selected component (if applicable):
At least Fedora 14 and newer.

How reproducible:
Always.

Steps to Reproduce:
1. Create a kickstart with 'raid /boot --fstype=ext4 --level=1 --device=md0 --useexisting'
2.
3.
  
Actual results:
Before Anaconda fully loads you get "Error parsing kickstart: Partitions required for raid"

Expected results:
Kickstart parses correctly, Anaconda installs system.

Additional info:
I've confirmed that everything works when these lines are removed.  I am not sure how to check for useexisting at that spot in the code.