EX-5102 utils: Add --pin option for 'lfs pcc attach' command

EX-5102 utils: Add --pin option for 'lfs pcc attach' command

If --pin option is provided, 'lfs pcc attach' command will
first pin then attach a file. So that user does not need
to run them separately.

Change-Id: I8e06af09abe7922399fd5e8e60132ff24ca31d1a
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial testlist=sanity-pcc
Reviewed-on: https://review.whamcloud.com/47067
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>