Lazily choose whether to skip tests

This is the new and improved version of an earlier PR for lazily determining whether a given test should be skipped. (notice the branch name: `skiptests3`) The decorators `skip_if_bz_bug_open` and `skip_if_rm_bug_open` have been created to replace `bzbug` and `redminebug`, respectively.

Many thanks to @elyezer for help with the decorators.
Aye, there's room for improvement in other ways too. For example, it would be cool if something in `tests/robottelo/` tested the code in `robottelo/common/decorators/`.

It's totally possible to create a single decorator, better test the code in the `decorators` module, and possibly find other ways to polish the `decorators` module. I'd be happy to go down that trail. That said, delaying this PR for the sake of that objective would push the fruits of this work back until at least next week.