LU-11503 tests: fix "division by 0" defect

LU-11503 tests: fix "division by 0" defect

if stop file is created before ha_repeat_mpi_load() or
ha_repeat_nonmpi_load() are started nr_loops is equal to 0
and test fails with
   (1476438773 - start_time) / nr_loops: division by 0
Patch fixes this case.

Patch correctly processes ha_dump_logs()->ha_on() rc
to avoid the test failure when nodes are not accessible for
logs dumping.

Test-Parameters: trivial
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-2205
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Change-Id: I97957875685492ddec66daa7c353ff6ef930933c
Reviewed-on: https://review.whamcloud.com/33350
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>