Create and test robottelo.orm.EntityDeleteMixin

Create a mixin class which provides inheriting classes with a `delete` method.
Under normal circumstances, this method returns either `None` or a foreman task
ID. Add tests for this class.
Make entities and tests use orm.EntityDeleteMixin

Insert `orm.EntityDeleteMixin` into the class hierarchy of several entity
classes. (e.g. Organization and Architecture) Update the following modules to
use the new `delete` method:

* `tests/foreman/api/test_multiple_paths.py`
* `tests/foreman/api/test_role_v2.py`
* `tests/foreman/api/test_system_v2.py`