Trim down test_multiple_paths.py

Make all all tested entities (i.e. classes in entities.py) inherit from
`EntityReadMixin`. Take advantage of this fact to trim down module
`test_multiple_paths`.

Add a logger to module `test_multiple_paths`. Log out information in several
tests, thus allowing the assertions to be trimmed down.
Add method `EntityReadMixin.read_raw`

`read_raw` makes a call to the server being tested and returns a
`requests.models.Response` object. It does not perform any validation, such as
checking the HTTP response code or decoding JSON.

Make `EntityReadMixin.read_json` call `EntityReadMixin.read_json`.
Remove 404 handling logic from `EntityReadMixin.read_json` and add this logic to
class `ActivationKey`. The 404 handling logic is only relevant to activation
keys.

Make module `test_multiple_paths` call `read_raw` instead of directly calling
`robottelo.api.client`.

Conflicts:
	robottelo/entities.py
	robottelo/orm.py
	tests/foreman/api/test_multiple_paths.py
Merge pull request #1684 from Ichimonji10/ak-404

Activation Key 404s
Skipping two api tests based on open BZs
- Skipping test_cv_promote_badenvironment_negative using bug 1091494. This bug is written for CLI but is reused here for API. A new bug will be created in future if need be.
- Skipping test_post_and_get using 1151240.  Earlier this was skipped using 1122267.  But 1122267 is closed in favor of 1151240
Merge pull request #1683 from sthirugn/zstream2

Skipping two api tests based on open BZs
Add log messages for virtual display feature

Sometimes the window manager process hangs and the test suite stuck. To
better find the cause was added logging messages to show when a virtual
display and a window manager is created, will be stopped and stopped.
The log messages will help debug the hanging problem and also will
provide more information about what is going on on.

Bonus: docstring formatting for robottelo.test module.
Force kill window manager if it hangs

Sometimes the window manager hangs and blocks the UI automation. Now the
window manager process will be force killed if it does not terminates in
30 seconds.

Also improve logging to show the window manager command as a string.
Kill window manager instead of trying to stop it

Fluxbox insists in not stop gracefully and the UI automation still
hanging. Because this, is better to send SIGKILL directly.
Merge pull request #1685 from elyezer/satellite-6.0.z.1

Cherry pick commits related to killing the window manager
Fix typo in UITestCase setUpClass method

Ask for cls.display instead of cls.window_manager when just the display
is defined.
Merge pull request #1686 from elyezer/satellite-6.0.z.1

Fix typo in UITestCase setUpClass method