Fix lemur local hadoop command construction

* In local mode, lemur ignored the `:step-jar` setting. Now it resolves
it using private step-jar function
* In steps using hadoop streaming, `:main-class` is omitted.
That was creating problems in local mode. 
Unfortunately, that does not work. 

The usage of the hadoop streaming command is 

     hadoop jar hadoop-streaming.jar [genericOptions] [streamingOptions]

Anything that fails to be parsed as a `genericOption` (like an empty string, in this case) will be passed on to the streaming option (e.g., `-input` or `-mapper`) parser, and you will get an `Unrecognized option` error in return.  

You can reproduce it locally by running

    hadoop jar /path/to/streaming.jar "" "-Dmapred.reduce.tasks=0"

Therefore the command cannot contain anything else but the name of the jar and a sequence of options.