Implement self.instances for all native providers

[jeffmccune]

Each of the native Puppet json providers lacks the instances class method, preventing the resource type from working with puppet resource and introspection. This also prevents prefetching from working within Puppet.

While working on the providers, also refactor the use of @conf to instead used @property_hash and @property_flush. This will bring the providers more in line with the official Puppet documentation, and other community resources like Gary's blog posts on the subject.

[ghoneycutt]

Closing as the new version of the module will likely not need this.