EX-3880 pcc: add pcc_async_affinity for async PCC attach

EX-3880 pcc: add pcc_async_affinity for async PCC attach

This patch adds a tunable parameter "llite.*.pcc_async_affinity"
that enables or disables the CPT selection in PCC-RO asynchronous
attach for testing.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I1473a7547555a2d6c615d37182b6cc359194aae0
Reviewed-on: https://review.whamcloud.com/45011
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>