Add backups_enabled for Digital Ocean salt-cloud provider

Digital Ocean profiles can now take 'backups_enabled: [True|False]'. (As of Jan 2014 this option must be specified on instance creation and can no longer be enabled for already-created droplets, so it's fairly necessary.)

Also added private_networking and backups_enabled to the documentation for the Digital Ocean provider; there was previously no mention of private_networking was an option, but it was in the code.

An incidental change is that private_networking won't default to False if not specified - by salt-cloud - but that is still the provider's default, and where the default should be coming from if a profile isn't explicit (i.e. there's no functional change but people will start getting private networking by default if Digital Ocean enables it by default).