LU-8544 test: using lfs df in client_up

LU-8544 test: using lfs df in client_up

With the patch http://review.whamcloud.com/#/c/19195/,
the LL_SBI_LAZYSTATFS option will be used during statfs call,
the "client_up" and "clients_up" in test-framework.sh should
be updated to use "lfs df" to determine the UP state of clients

Change-Id: I124e3ecc288f04b32dfd286f48e0ab75b1385a29
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/22459
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>