remove _check_container_exists from Swift backup driver

The HTTP PUT on a Swift container is idempotent, we don't need to
check if it exists before uploading a backup.

This fixes a side issue where swiftclient was printing a TRACE of
the failed HTTP HEAD, previously in use to check if the container
existed.

Change-Id: I7bc267c948b56e30658997a9743f0d46ebb362ab
Merge "remove _check_container_exists from Swift backup driver"