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.
Merge pull request #10 from Ichimonji10/drop-func

Drop read_config_file, refactor child_register
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`
Refactor function child_copy_repo

Make function more compact. It now contains only six statements, compared to
nine previously. Make function `main` only call `child_copy_repo` if `REPO_FILE`
exists. Example output where `REPO_FILE` exists ("…" stands in for the host
name):

    >>> from katello_rapidnode import child_copy_repo
    >>> child_copy_repo('…')
    Copying myrepofile.repo to …...
    >>>
Merge pull request #12 from Ichimonji10/repo-file

Refactor function child_copy_repo
Update documentation

Rewrite the README. Expand it and attempt to more clearly outline how to use
katello-rapidnode.