EX-5128 pcc: avoid to specify attach ID for every pin

EX-5128 pcc: avoid to specify attach ID for every pin

In this patch, it avoids the need to specify "-i <attach_id>" for
every pin/unpin operation as in the very common case there is only
a single cache for that client.
If attach ID is not specified, it will select the first dataset
on the client as PCC backend.

Add the test case: sanity-pcc test_205.

Change-Id: I04e2b3d9ca360ae40f3723d7cd9bd78b22602123
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/47105
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng, Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>