LU-8950 tests: standardize file creation in sanity-hsm

LU-8950 tests: standardize file creation in sanity-hsm

sanity-hsm exposed several functions to create files:
 - make_small()
 - make_small_sync()
 - make_archive()

This patch renames all these functions (respectively):
 - create_small_file()
 - create_small_sync_file()
 - create_archive()

And adds a new one: create_empty_file() which creates an empty file

The actual file_creation is now abstracted in the create_file()
function used by all the aforementionned functions.

Test-Parameters: trivial testlist=sanity-hsm
Signed-off-by: Quentin Bouget <quentin.bouget@cea.fr>
Change-Id: Ib4267827eb2e31ed396da1d711419f45f9e0ef16
Reviewed-on: https://review.whamcloud.com/24704
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>