Description of problem:
spacewalk-repo-sync fails if the parent of a custom channel (with a repo associated with it) is not also a custom channel.

How reproducible:
Always

Steps to Reproduce:
1. create a custom channel (with repo) that is a child of a Red Hat channel. In this case, I created epel-6-x86_64 that is a child of rhel-x86_64-server-6
2. spacewalk-repo-sync --channel epel-6-x86_64 --type yum

Actual results:
Traceback (most recent call last):
  File "/usr/bin/spacewalk-repo-sync", line 187, in <module>
    sys.exit(abs(main() or 0))
  File "/usr/bin/spacewalk-repo-sync", line 107, in main
    l_ch_custom=reposync.getCustomChannels()
  File "/usr/lib/python2.6/site-packages/spacewalk/satellite_tools/reposync.py", line 99, in getCustomChannels
    d_parents = getParentsChilds()
  File "/usr/lib/python2.6/site-packages/spacewalk/satellite_tools/reposync.py", line 93, in getParentsChilds
    d_parents[parent_channel].append(row['label'])
KeyError: 'rhel-x86_64-server-6'

Expected results:
We should be able to sync the repos associated with epel-6-x86_64 even though its parent is not a custom channel.