Fix LogstashFormatter initializer

The current initializer takes only the first value provided in the "keys" parameter, and ignores all others.
With the fix, the formatter can be initialized with e.g.
{
   '()': 'canary.format.LogstashFormatter',
   'keys': ['fields', 'name', 'levelname'],
}
To add more information to the records being sent to logstash.