LU-14273 tests: enhance ha.sh to run custom cmd on bg

LU-14273 tests: enhance ha.sh to run custom cmd on bg

We need this ability to run lfs migrate in parallel
with mdtest and IOR.

Test-Parameters: trivial
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9371
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Change-Id: Id52ed0731aba24d3f40813da5fd2bb9b94ae63e5
Reviewed-on: https://review.whamcloud.com/41073
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andriy Skulysh <askulysh@gmail.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>