Tweak global settings

Make the `server.hostname` field in `robottelo.properties.sample` blank by
default. This prevents accidental battering of any domain by Robottelo users.
Formerly, a user could fetch a copy of the codebase and unwittingly start
battering awesome.server.com; now, that cannot happen.

Make the .gitignore file ignore files in a more targeted manner.

* The `_build` folder will only exist in `/docs/_build/`. Therefore, the full
  path to the `_build` folder should be used so as to avoid accidentally
  ignoring other dirs with the same name.
* The `docs/tests.*` and `docs/tests.*` files were formerly generated by
  sphinx-apidoc. However, that tool is no longer being used.