Reset target_id when "None" is selected in Type pull-down.

- Reset target_id key to nil when target_class selection is set to none.
- Added spec test to verify value of target_id key

https://bugzilla.redhat.com/show_bug.cgi?id=1097876
Merge pull request #340 from h-kataria/ae_tools_simulation_screen_fix

Reset target_id when "None" is selected in Type pull-down.