Fixed few failing tests
Add upload and delete manifest utility methods

These methods help manage an organization's subscription thought API by
allowing upload or delete a manifest.

Also add tests for paths and delete_manifest helper method. Other helper
methods should have their tests when the requests wrapper gets fixed.
Merge pull request #1256 from Ichimonji10/docs

Add module to docs, fix up module
Merge pull request #1249 from sghai/fix-tests

Fixed few failing tests
Merge pull request #1255 from elyezer/api-manifest-management

Add upload and delete manifest utility methods
Add class robottelo.orm.EntityReadMixin

Create a mixin class named `EntityReadMixin`. This mixin adds a method named
`read` to classes that use it. The `read` method returns an entity object - not
merely a dict of data!

Fix up a few issues with the docstring for `EntityDeleteMixin`.