Add common code for fileutils.

This code is present in nova, cinder and also needed by ironic package.
* delete_if_exists
* remove_path_on_error

Also added some tests.

openstack/common/fileutils.py => nova.common.utils <=> cinder.common.utils

Change-Id: Id5001db161bb8e7dfb20d1b2cc2033e886cda32f
Implements: blueprint image-tools
Merge "Add common code for fileutils."