LU-177 lbuild autonomy fixes

LU-177 lbuild autonomy fixes

Fixes to allow lbuild to work without uneeded help from it's caller:
- kernel srpm location for rhel5
- CentOS is a "rhel" distro
Direct output to the output FD.
Refactor find_linux_rpm-rhel{5,6}.

Signed-off-by: Brian J. Murrell <brian@whamcloud.com>
Change-Id: I714b90b53b14ac4ef4cd6bfe638a029656a151b0
Reviewed-on: http://review.whamcloud.com/361
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Michael MacDonald <mjmac@whamcloud.com>