Switch to the oslo_utils.fileutils

Switch to the oslo_utils.fileutils

fileutils is graduated in the oslo.utils library.
Use the open function from builtins instead of fileutils.file_open.
Change the test_backup_volume (test_scality module) and
test_restore_backup (test_scality, test_volume) functions to use
mock library instead of mox.

Implements: blueprint graduate-fileutils[1]
[1] https://blueprints.launchpad.net/oslo-incubator/+spec/graduate-fileutils

Change-Id: I7abf0273bf2b90c72f51ebf4896808eaabb91969