EX-3108 build: update kernel to -ddn13

EX-3108 build: update kernel to -ddn13

Update the kernel version to -ddn13 to match the version used
on b_es5_2 so that it is possible to just upgrade the Lustre
RPMs when moving from EXA5.2.2 to EXA6.0.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I8845fa2c797769b94971e60dc92cdfb2c79bb570
Reviewed-on: https://review.whamcloud.com/43534
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: Minh Diep <mdiep@whamcloud.com>