LU-14586 tests: set mpi np correctly

LU-14586 tests: set mpi np correctly

The number of mpi processes is to be calculated
based on the number of clients in clients subset.

Fixes: 9ecb000 ("LU-13281 tests: ha.sh improvements")
Test-Parameters: trivial
Signed-off-by: Elenai Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9716
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: If574743e2e29a309a8d7a021056fa726495fa959
Reviewed-on: https://review.whamcloud.com/43217
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>