LU-10465 lov: increase default stripe size to 4MB

LU-10465 lov: increase default stripe size to 4MB

Increase the default stripe size from 1MB to 4MB
so that widely-striped files can generate full RPCs
without pinning so much memory on the client.

The patch also renames STRIPE_BYTES and STRIPES_PER_OBJ
to DEF_STRIPE_SIZE and DEF_STRIPE_COUNT in cfg/local.sh,
and unsets them to support formatting Lustre filesystem
with default stripe size and count.

Change-Id: I59d1fdb3e30599c125e0e5e800d168921bd69098
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/27151
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>