LU-10664 tests: fix MPI tests in dom-performance.sh

LU-10664 tests: fix MPI tests in dom-performance.sh

Make MPI tests in dom-performance.sh to be ran under
mpiuser instead of root

Test-Parameters: trivial mdssizegb=20 testlist=dom-performance
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ief3036434191a0bc153d5c8e380183d5e5067dc4
Reviewed-on: https://review.whamcloud.com/37044
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>