EX-3730 pcc: add test for concurrent read from 2 clients

EX-3730 pcc: add test for concurrent read from 2 clients

This patch add a test case with concurrent read access from 2
clients.
The purpose is to verify that the client will not re-attach file
into PCC backend once attached when the file is read access
concurrently from 2 mount points on a client according to the
PCC attach stats.

This patch also fixes the help message for PCC-RO.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ibb038bd3a74f43031b6fab4e65565620c416909e
Reviewed-on: https://review.whamcloud.com/44791
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>