RepositorySet-API: Added the functionality to enable RedHat Repos.

Added `list_rhproducts` function to list all the RedHat Products
in a given Organization.

RH Products have varying Id's depending upon various other custom
products, hence we use the product-label which is a static value
to fetch the RH Product's ID in `fetch_rhproduct_id` function.

RH Reposets too have varying ID's hence we use reposet-label which
is a static value to fetch the RH Reposet's ID in `fetch_reposet_id`
functio.

Also RH Repositories first need to be enabled , so that they can be
synced.
`enable_repo` and `disable_repo` functions requires the
`reposet_id`, `basearch` and `releasever` to enable, disable the
repositories.
There _should_ only be one result, but what if multiple results are returned? How about returning an exception? Even better, how about changing this method and making it return a list of IDs?

```python
return [result['id'] for result in response.json()['results']]
```