sam to bam not working in workflow editor

The reference genome is not displayed in the workflow editor. So I guess this needs to be a special cased in the editor. This is what we get after running the workflow. Rereunning the crashed tool works.

```
job traceback:
Traceback (most recent call last):
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/jobs/runners/__init__.py", line 166, in prepare_job
job_wrapper.prepare()
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/jobs/__init__.py", line 873, in prepare
tool_evaluator.set_compute_environment( compute_environment, get_special=get_special )
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/tools/evaluation.py", line 56, in set_compute_environment
self.tool.handle_unvalidated_param_values( incoming, self.app )
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/tools/__init__.py", line 1756, in handle_unvalidated_param_values
self.handle_unvalidated_param_values_helper( self.inputs, input_values, app )
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/tools/__init__.py", line 1775, in handle_unvalidated_param_values_helper
self.handle_unvalidated_param_values_helper( input.cases[current].inputs, values, app, context, prefix )
File "/usr/local/galaxy/galaxy-dist/lib/galaxy/tools/__init__.py", line 1796, in handle_unvalidated_param_values_helper
raise LateValidationError( message )
LateValidationError
```

Removing the validator from the tool fixes it, but this is not the way to do :)

ping @guerler