Fixing RHN sync and adding checksum option

The commit 8201447 fixes the RHN sync issues I encountered where arguments were passed in wrong order, certain keys were missing and an old version of Satellite was reported to RHN.

The commit 7608d38 adds the possibility to choose the checksum type for each RHN channel. If not defined it defaults to sha256. For example:

/etc/grinder/grinder.yml:
-   label: rhel-x86_64-server-5
    relpath: rhel/5/server/
    checksum: sha

-   label: rhel-x86_64-server-6
    relpath: rhel/6/server/