Checkbox dependencies sometimes fail when dependents aren't available

Sometimes, the depends attribute in a test definition might point to a test name which either doesn't meet requirements or simply doesn't exist. The current behavior produces the following traceback which results in no tests being run. Instead, only that test should be skipped.

Traceback (most recent call last):
  File "/srv/trunk.certification.canonical.com/checkbox/checkbox/reactor.py", line 89, in fire
    results.append(handler(*args, **kwargs))
  File "/srv/trunk.certification.canonical.com/checkbox/plugins/tests_prompt.py", line 87, in prompt_tests
    self._iterator = JobIterator(tests, self._manager.registry)
  File "/srv/trunk.certification.canonical.com/checkbox/checkbox/job.py", line 103, in __init__
    iterator = DependsIterator(iterator, compare)
  File "/srv/trunk.certification.canonical.com/checkbox/checkbox/depends.py", line 41, in __init__
    depends = [element_dict[d] for d in depends]
KeyError: 'cd-dvd/cdrom-read'