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.
Will righ the testcases for enabling the rhreposets soon, until then, this should work, if manifest is imported in sat6.

```python
prd_id = entities.Organization(id='1').fetch_rhproduct_id('Red Hat Enterprise Linux Server')
reposet_id = entities.Product(organization='1', id=prd_id).fetch_reposet_id("Red Hat Enterprise Linux 6 Server - RH Common (RPMs)")
task = entities.Product(organization='1', id=prd_id).enable_rhrepo(reposet_id, 'x86_64', '6.5')
```