LU-2687 test: add b2_4 zfs image for conf-sanity test_32a

In order to ensure that we do not break ZFS upgrades
in the future, add 2.4.0 zfs filesystem test image for
conf-sanity.sh test_32a.

Test-Parameters: mdtfilesystemtype=zfs \
ostfilesystemtype=zfs mdsfilesystemtype=zfs \
envdefinitions=SLOW=yes testlist=conf-sanity

Change-Id: Iae560e05b428907409dc7069d30b601b52750cca
Signed-off-by: Wei Liu <wei3.liu@intel.com>
Reviewed-on: http://review.whamcloud.com/7193
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>