Drop dependency on fileutils from oslo db tests

Module fileutils from openstack common requres common log.
We should avoid of use openstack common log module, because it
registers a few oslo.config options, so can possibly lead to
unpleasant issues (DuplicateOptError).

We used only fileutils.write_to_tempfile() function, from fileutils,
so it can be replaced to similar create_tempfiles() method from
BaseTestCase() class.

Related to blueprint oslo-db-lib

Change-Id: I25d97c877687a8b5e4343f5376e92ba269bca98a
Merge "Drop dependency on fileutils from oslo db tests"