cu2d-trigger should fail gracefully if no projects are defined in stack

[Martin Mrazik]

This is what happens when no projects are defined in the stack:

2013-04-03 17:05:26,216 DEBUG Loading stack configuration from ../stacks/raring/misc.cfg
Traceback (most recent call last):
  File "./cu2d-trigger", line 39, in <module>
    sys.exit(command(BASEDIR))
  File "./../c2dconfigutils/cu2dTrigger.py", line 142, in __call__
    for trigger in trigger_list:
UnboundLocalError: local variable 'trigger_list' referenced before assignment