LU-2420 tests: add add_group() and add_user()

LU-2420 tests: add add_group() and add_user()

This patch adds add_group() and add_user() functions into the
test-framework.sh. They are used by setup_posix_users() in
posix.sh.

This patch is backported from commit 34a6092 for LU-1661.
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I33baec4abe60752ba236205e03d607a5da250216
Reviewed-on: http://review.whamcloud.com/4797
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>