EX-3292 pcc: avoid to specify ID for every attach

EX-3292 pcc: avoid to specify ID for every attach

In this patch, it avoids the need to specify "-i <attach_id>" for
every attach 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.

And the new format of PCC state is as follows:
file: /mnt/lustre/f42.sanity-pcc, type: readonly, PCC_file:
/d42.sanity-pcc/0402/0x200000401:0x3:0x0, open_count: 0, flags: 0

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Icd23eda5dca4711f9bb7af940f6cef5ddb97ce69
Reviewed-on: https://review.whamcloud.com/43946
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Feng, Lei <flei@whamcloud.com>