EX-4006 pcc: make "pccro=1" default

EX-4006 pcc: make "pccro=1" default

To avoid a risk that users will accidentally configure PCC-RW and
potentially lose data if those client nodes go offline, this patch
makes "pccro=1" default for PCC backends.

This patch adds a new option "--w|--write" for PCC-RW cache
mode when attach a file.
Also It makes "--r|--readonly" as a default option for PCC attach
command.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I56735b0ebe8f0d9ef22b3f7e39e8cccfa3aad443
Reviewed-on: https://review.whamcloud.com/45162
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>