Support :postgres Persist Plugin

Seems like the way to accomplish this is by mirroring the attributes in the updated `conf/razor_server.conf` file. Currently, my proposal is to add the following attributes:

```ruby
node.default['razor']['persist_mode'] = "mongo" # could also support `"postgres"`
node.default['razor']['persist_host'] = "127.0.0.1"
node.default['razor']['persist_port'] = 27017
node.default['razor']['persist_timeout'] = 10
```

The `:persist_mode` attribute logically conflicts with the existing `:mongodb_address` attribute, so it should be dropped or at least deprecated with a warning log message and note in the README.