Improve CLI direct commands tests
Add tests on Cobbler CLI object actions
Improve create and get tests

* Add system fields to be tested
* Add get tests
* Improve error msg when modify_* fails
Restructure XML-RPC tests

* Restructure tests: remove dependencies among unittest subclasses.
  All tests in a unittest subclass now are self-contained
* Add more tests
* Add documentation
Remove old tests
Merge CLI test modules
Move new Cobbler tests to tests directory
Merge pull request #970 from alanoe/alanoe/refactor_tests

alanoe/refactor tests