LU-4059 tests: Archive access must be done from agent

In sanity-hsm test_11, the size of an archived file is
requested from a client and not necessarily from an agent.
So, this patch now makes the call to stat from an agent.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I2f7ef294a0e9e28301312ad8529c6178279f5c19
Reviewed-on: http://review.whamcloud.com/7851
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>