LU-1762 tests: get correct MMP update and check intervals

This patch fixes the get_mmp_update_interval() and
get_mmp_check_interval() in mmp.sh to get the correct
MMP update and check intervals from both the old and
new outputs of debugfs.

The patch also improves test_8() to increase the running
time of e2fsck to allow mount operation to be started
before e2fsck operation stops.

Test-Parameters: testlist=mmp
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: If614659e3b7fe45c4b406d5541b1c2944b3c37ce
Reviewed-on: http://review.whamcloud.com/4953
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>