EX-5176 pcc: use JSON string for trusted.pin xattr

EX-5176 pcc: use JSON string for trusted.pin xattr

Change the format of trusted.pin xattr to JSON.
For example:
trusted.pin=[{"hsm":2}]
trusted.pin=[{"hsm":2},{"hsm":3},{"pool":"ddn_ssd"}]

Change-Id: I38aeb4ad960cf8324e77ac467f5e5cd742f9ec86
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial testlist=sanity-pcc
Reviewed-on: https://review.whamcloud.com/47161
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>