Drop prefix on yumserver-related class parameters.

Example: Change $cdh_yumserver, $cdh_yumpath, and $cdh_version to
$yumserver, $yumpath, and $version.
Clean out commented code.
Collapse class anchors.
Simplify the $use_parcels conditional.

Move Class['cloudera::cm::repo'] out of the conditional.
Do not start CM agent until all software is present.
Require Class['epel'] on EL5 for LZO/GPL Extras.
Update documentation.
Call apt::source with $architecture where appropriate.