Add convenience functions for working with the API

Add functions to get repository's errata and packages
Add robottelo.api.utils module to docs
Merge pull request #1005 from elyezer/api-utils

Add convenience functions for working with the API
Make better error messages

Call a function that builds an error message whenever any tests fails in module
tests.foreman.api.test_multiple_paths fails. This allows the testing code itself
to be more concise and for error messages to be more consistent.
Use a content-type of application/json

By default, use a content-type fo application/json whenever sending a request
via module `robottelo.api.client`. If the content-type of a message is
application/json, also encode the data in that message as json. If the user
explicitly sets the content-type of a message, do not override that setting.

Add tests to ensure this code is correct.
Refactor existing tests and docstrings.

Use the ddt module to slim down existing tests. Rewrite some docstrings to
improve intra-documentation links.
Update module docstring