Use fmt to specify source_host and extra

When using logging.config.fileConfig (and maybe also dictConfig) it's
not possible to specify parameters to the formatter. Since
logstash_formatter isn't using the fmt/format parameter we can (ab)use
it to specify source_host and extra. This enables us to make use of
source_and and extra in fileConfig. fmt is just a json string. For
example:

[formatter_logstash]
class=logstash_formatter.LogstashFormatter
format={"source_host": "test.example.com", "extra": {"a": "123", "x":
"abcd"}}