LU-1661 tests: improve setup_posix_users()

LU-1661 tests: improve setup_posix_users()

This patch improves setup_posix_users() in posix.cfg to use
do_rpc_nodes to add groups and users on remote nodes,
so as to simplify add_group() and add_user() and make them
become common functions to be used by other test scripts.

Test-Parameters: clientdistro=el5 testlist=posix
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: Ie07f32dffe7d7ae2037d92bab372977efff2e5fe
Reviewed-on: http://review.whamcloud.com/3770
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>