Change return type of Configuration#getInt to int

The org.graylog2.plugin.configuration.Configuration class only stores values with
the precision of int anyway (even if the input type was long or double) so we don't
lie about the underlying return type.
Fix port configuration of GelfOutput

The `port` setting is a NumberField and will be stored as Integer in the output's
configuration map.
Merge branch '0.90' into 0.91