Make main(), drop pylint directives and some funcs

Make a `main` function at the very top of the `katello_rapidnode` module. Push
logic from the bottom of the module in to this function. Place this at the
bottom of the module instead:

    if __name__ == '__main__':
        main()

It should now be possible to import the `katello_rapidnode` module without
triggering side effets like, uh... setting up a capsule. This should make it
possible to create unit tests.

Drop several `pylint:disable` directives:

1. W0621 (redefining-name-from-outer-scope)
2. W0632 (possible-unbalanced-tuple-unpacking-with-sequence)

The first issue is fixed by moving logic in to function `main`. The `child`
variable no longer conflicts with the `child` argument to several functions. The
second issue was fixed by f4cbc54.

Drop the `get_parent` and `get_children` functions. They are extremely
simplistic single-use functions.