core: [ExternalTasks] Cannot end existing job

Enable to set the JobId or Step Id as parameters to commands that are
not monitored but handle the External Job/Tasks :
EndExternalJobCommand
ClearExternalJobCommand
EndExternalStepCommand

Change-Id: I78f72cd619e32e5763f46391932ec70730a2b4b7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988094
Signed-off-by: Eli Mesika <emesika@redhat.com>