Make travis run tests in `tests/robottelo/`

The test suite will fail unless the packages listed in `requirements.txt` are
installed. Make Travis install them.

Also add a comment explaining why the tests in `tests/foreman/` are not run.
Remove unnecessary static vars

This fix allows the relevant class to be imported even when no config file is
present, and therefore fixes one of the errors issued when running unit tests
in `tests/robottelo/`. Also, it's kinda silly to add static vars to a class for
no good reason.
Fix reference to _properties

Fix all references to the instance variable `Config._properties`. Also populate
the config file dictionary with enough values for the relevant test to proceed.
Make API docs reference recently added module
Reorder API documentation content

If a package contains any sub-packages, links to those sub-packages should
appear at the very top of the relevant documentation. This is in line with user
expectations: navigation elements typically appear above content.