template-ify create_backup.py to set backup URI based on endpoint

files/default/create-backup.py connects to 'backup.api.rackspacecloud.com', which is an ORD URI. The file needs to be generalized to allow other region's URIs, e.g. iad.backup.api.rackspacecloud.com, dfw.backup.api.rackspacecloud.com, etc.

If you run create-backup.py as is on, say, an IAD-only customer, it will fail to create the backup plan.

This pull request should be ready to merge, but note: Any customers using this rolebook who are primarily in ORD will likely need to have the Cher attr node['rackspace_cloudbackup']['rackspace_endpoint'] set to 'ord' in the rolebook or chef environment prior to upgrading to this merge. Otherwise they'll get dfw.backup.api.rackspacecloud.com in create-backup.py since 'dfw' is the default endpoint set in attributes.rb.