Add backup_swift_auth_url to swift backup driver

Add backup_swift_auth_url to swift backup driver

Currently the config parameter backup_swift_url expects a
keystone endpoint url in the swift backup driver if you're
using single_user mode, but if you're in per_user mode it
expects a swift endpoint url. This patch adds a parameter
called backup_swift_auth_url to differentiate the two.

Unit tests are also provided for testing
backup_swift_auth_url.

DocImpact: The config options "backup_swift_auth_url" and
"keystone_catalog_info" will need to be added to the Cinder
docs.
Change-Id: Iee466fe1404eef9402a19d529dba19c7d5a4ef2b
Closes-Bug: #1449972