LU-2861 tests: use mkfs_opts() instead of *_MKFS_OPTS

LU-2861 tests: use mkfs_opts() instead of *_MKFS_OPTS

This patch fixes conf-sanity.sh to use mkfs_opts()
instead of *_MKFS_OPTS for adding new server targets.

This patch also ports the conf-sanity.sh changes from
commit 648b69c to support the interoperability with
2.4 server.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes \
clientdistro=el6 serverdistro=el6 \
serverjob=lustre-master serverbuildno=1297 \
testlist=conf-sanity

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I62d99ef70ada044eeda6f873fb835e603dc48df2
Reviewed-on: http://review.whamcloud.com/5587
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>