Add backups_enabed option to Digital Ocean create().

- Group backups_enabled and private_networking, the two binary options.
- private_networking now absent if excluded rather than 'false'.
- Deal with non-binary arguments explicitly as other providers do.

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.)

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.