Fix Sphinx warnings

* Do not reference the non-existent modules `robottelo.factory` and
  `tests.robottelo.test_factory`.
* Do not reference the non-existent method `robottelo.factory.Factory.build`.
* Correctly demarcate a code block with `::`.
* Change a mistaken reference from `gen_missing` to `create_missing`.
Merge pull request #1691 from Ichimonji10/sphinx

Fix Sphinx warnings