LU-4340 tests: Fix test_69 of conf-sanity test

LU-4340 tests: Fix test_69 of conf-sanity test

Since 100% inodes were used with default value of MDSSIZE and OSTSIZE,
test_69 of conf-sanity test was failing with ENOSPC error.

Use decreased value of num_create to create 20000*3 files.

Also added umount of client before OST replace operation so that
eviction/recovery of client is not seen.

Seagate-bug-id: MRP-1698
Signed-off-by: Ashish Purkar <ashish.purkar@seagate.com>
Change-Id: I4568cfb3d259ae248f26d5463cd3291db60f2d5b
Reviewed-on: http://review.whamcloud.com/15487
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>