shell: better handle job output parameters

Due to openlmi's workaround of pegasus' shortcoming, JobOutParameters
property of job instance is something else then it should be. It's an
instance of __MethodParameters_<method_name>_Result class which derives
from __MethodParameters_<method_name> class. The latter contains input
parameters of asynchronous method. Therefor JobOutParameters may contain
input parameters as well. Let's filter them out because this may confuse
client scripts.