Fix ChunkedBackupDriver _create_container

Fix ChunkedBackupDriver _create_container

Current _create_container method in ChunkedBackupDriver is not behaving
as expected.

According to update_container_name docstring the method should return
None if the container name that comes in to the driver is to be used,
but code was using backup_default_container instead.

Also, if None is returned by update_container_name the logging will
report that it will be using None instead of the name that it will be
really using.

If update_container_name returns the same value we already have in the
DB, driver will unnecessarily update the DB.

This patch fixes all this.

Change-Id: I2aeac37f5533b6bf0c10dd6bfe45224a25f39ea6
Closes-Bug: #1534182