LU-2548 quota: proper iam index truncation

In quota migration, the iam container needs be reinitialized
after truncation.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I622cc8e4e095cc0c9dcd2d2ce0e245a6de376b3d
Reviewed-on: http://review.whamcloud.com/5292
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>