lbuild: seperate kernel tag from lustre version

lbuild: seperate kernel tag from lustre version

Bumped Kernel tags using different tags like Lustre.
As Lustre changed more frequently than Kernel changes.

With this change, we only bumped DDN kernel tag
if there is really some kernel changes here.

Change-Id: Icdebe62fe1ef5de2096ae0f568494db394dc04f7
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/41827
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>