Sync filetuils from oslo-incubator

to support https://review.openstack.org/#/c/33409/

* nova/openstack/common/fileutils.py: Auto sync from oslo.
* nova/tests/virt/libvirt/test_libvirt.py (fake_rm_on_errror):
Adjust to match the new signature of delete_if_exists()
and handle mocking out the remove parameter to
remove_path_on_error() which is initialized at load time.

Change-Id: I756f1226853366e8f418d21309b0e8dcf8a24aa2
Merge "Sync filetuils from oslo-incubator"