LU-7859 tests: Use getent instead of /etc/passwd

LU-7859 tests: Use getent instead of /etc/passwd

Use getent passwd instead of directly reading form /etc/passwd
to prevent missing user checks if users come from ldap or another
auth source that isn't /etc/passwd.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ia2831c777c7c3f8bae5a634fa897744954d14c1e
Reviewed-on: http://review.whamcloud.com/18846
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>