Fix config file parsing

Add `configparser` to the list of requirements. ConfigParser saw a major update
in Python 3.2, and this module backports those changes to Python 2.6.  The
default configparser module is used in Python 3.2+, and the backported version
is used in older versions of Python.

As a side-effect of this change, drop a conditional import and do not bother
inspecting `sys.version_info`.
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)
3. R0914 (too-many-local-variables)

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 f4cbc54ae61d77837ce6e73455bba1f47898c75a. The third
warning is still a legitimate issue, and uncommenting the warning reinforces the
fact that the code has design issues.

Drop the `get_parent` and `get_children` functions. They are extremely
simplistic single-use functions.
Drop read_config_file, refactor child_register

Drop function `read_config_file`. It does nothing more than return a tuple of
values from the config file. The contents of the config file are available to
all functions via the global variable `CONFIG`, so this extra level of
indirection is not necessary.

Refactor function `child_register`. It made the most extensive use of
`read_config_file` via this line of code:

    adminpassword, orgname, contentview = read_config_file()[4:7]

It was also a bit messy in general.

Both the Python 2 and 3 versions of flake8 and pylint are happy with the script.