Symbolize kyes in `config` before accessing it

I found that sub keys in `config[:varz]` are Strings when the hash passed from bin/cf-registrar, because it is read from a config file. UAA fails to send varz credentials because of this bug.

Here is the error log in /var/vcap/sys/log/uaa/cf-registrar.log:
```
{"timestamp":1377275176.0350683,"message":"Could not register nil varz credentials","log_level":"error","source":"cf.registrar","data":{},"thread_id":13992060,"fiber_id":16611160,"process_id":22933,"file":"/var/vcap/data/packages/uaa/28.1/vcap-common/lib/cf/registrar.rb","lineno":48,"method":"register_varz_credentials"}
```