LU-12399 tests: avoid 'pdsh localhost' in sanity test_420

LU-12399 tests: avoid 'pdsh localhost' in sanity test_420

sanity test_420 needs a clean env to execute openfile, ie not
inherited from root user.
Replace 'pdsh localhost' with simpler 'su - $uname -c' alternative
to achieve this.

This patch is back-ported from the following one:
Lustre-commit: 1476ac047b449886a0c382b840a7b09dc0cec7eb
Lustre-change: https://review.whamcloud.com/35176

Test-Parameters: trivial envdefinitions=ONLY=420 testlist=sanity
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ifeba7fc1eba86d74a64cca187e286adb23147e2e
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35250
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>