Replace decorator bzbug with skip_if_bz_bug_open

Decorator skip_if_bz_bug_open can be used just like bzbug. However, there are
several advantages to using the new decorator.

* The new decorator is better named.
* The new decorator does not trigger calls to a bugzilla server when it is
  evaluated. Instead, calls to a bugzilla server do not occur until the test
  suite is executed.
* The new decorator skips tests under slightly different conditions. Formerly,
  if an error occurred while fetching information about a Bugzilla bug, the
  test would be skipped. Now, the test is allowed to run.

Finally, a new method get_bugzilla_bug has been created. It is reponsible for
fetching and caching bug objects.