EX-3054 lipe: unlink pcc cache file if it's needed

EX-3054 lipe: unlink pcc cache file if it's needed

Cache file is not guaranteed to be deleted after detach
operation returns success. So we unlink the cache file
if it's needed.

Change --force_clear option to --clear_hashdir option.
It tries to remove empty hash dir recursively up to the cache
dir root. The option is off by default.

Change-Id: I49911c688faaf6c7baa814b260a4ff492de077fc
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Reviewed-on: https://review.whamcloud.com/43403
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>