LU-8817 tests: Update 'min_ost_size'

LU-8817 tests: Update 'min_ost_size'

On setup where MDS and client are on same node
lctl get_param osc.*.kbytesavail returns the
value from OSP. This patch updates function
min_ost_size to use "lfs df" output instead of
reading it from proc.

Output of 'lfs df'. 'Available' field only.
$ lfs df | grep OST | awk '{print $4}' | sort -n | head -n1
151276

Output with current code:
$ lctl get_param -n osc.*.kbytesavail | sort -n | head -n1
151004

Test-Parameters: trivial testlist=sanity-benchmark,obdfilter-survey
Signed-off-by: Arshad Hussain <arshad.hussain@seagate.com>
Change-Id: I07ddff43d7fc4b39b89bd998f96bf36776c5c048
Seagate-bug-id: MRP-3683
Reviewed-by: Ashish Purkar <ashish.purkar@seagate.com>
Reviewed-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Tested-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Reviewed-on: https://review.whamcloud.com/23685
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Ujjwal Lanjewar <ujjwal.lanjewar@seagate.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>