Add unit tests for function `parent_get_capsules`

Also clean up function `parent_get_capsules`. The new unit tests are basic, but
they do prove that it is possible to import module `katello_rapidnode` without
negative side effects and create unit tests for the code therein.
Merge pull request #11 from Ichimonji10/unit-test

Add unit tests for function `parent_get_capsules`