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`
Make EntityDeleteMixin.delete synchronous

When an entity is deleted, an HTTP 202 response may be generated, indicating
that a task for that action has been created. Make `EntityDeleteMixin.delete`
look for an HTTP 202 response and, if one is received, wait for the task to
complete.

As part of this refactoring, create a helper function which polls the server,
and make both `orm.EntityDeleteMixin` and `entities.ForemanTask` use this helper
function.
Add a test subscribing a system to a content view

Add a new test module: `tests.foreman.api.test_contentview_v2`. Add a test to
this module which does the following:

1. Create an organization, lifecycle environment and content view.
2. Publish the newly created content view.
3. Promote the newly published content view.
4. Create a system which references the organization, lifecycle environment and
   content view.

Update the following methods:

* `ContentView.publish`
* `ContentViewVersion.promote`

Make these two methods raise an exception if an HTTP 4XX or 5XX response is
received, or return decoded JSON data otherwise. Update all usages of those
methods.
Reword the readme file for the sake of clarity

Rewrite prose in the readme section "Running UI Tests On a Headless Server" for
the sake of clarity and brevity.
Merge pull request #1247 from Ichimonji10/wordsmithing

Reword the readme file for the sake of clarity
Add system registration stubs for CLI and UI
Merge pull request #1231 from elyezer/system-registration-stubs

Add system registration stubs for CLI and UI
Merge pull request #1237 from Ichimonji10/test-subscribe-system

Add a test subscribing a system to a content view
Merge pull request #1239 from Ichimonji10/entity-delete-mixin

Create, test and use EntityDeleteMixin