Drop custom error checking code

Drop custom error checking code from `EntityFactoryMixin.create`. Drop class
`factory.FactoryError` and reference `requests.exceptions.HTTPError` instead as
necessary. Update and simplify unit tests for the `factory` module.
Merge pull request #1441 from Ichimonji10/simplify-factory

Drop custom error checking code
Clean up method `skip_if_sam`

* Reword the docstring with nicer prose.
* Change variable names for the sake of clarity.
* Use a list comprehension instead of a map-and-lambda combo.

A list comprehension is used instead of a map-and-lambda combo because:

* List comprehensions are more common, and in my opinion more readable. When in
  Rome...
* There is anecdotal evidence that list comprehensions are preferred by the
  community at large. See the comments to PEP 279.
* Pylint complains about the usage of a map-and-lambda combo when a list
  comprehension suffices.
* Last and least, list comprehensions are more efficent.
(API) Fix up the "Architecture" entity

Override `Architecture.create` and `Architecture.read`. This makes it possible
to:

* Create an architecture that points to some number of operating systems.
* Read which operating systems an architecture points to.

Update the test suite to exercise these changes.
Merge pull request #1445 from Ichimonji10/refactor

Clean up method `skip_if_sam`
Merge pull request #1444 from Ichimonji10/architecture

(API) Fix up the "Architecture" entity
UI sam fencing
Removing API reference due to unavailability
Merge pull request #1449 from JacobCallahan/master

Removing API reference due to unavailability
Removing the broken link for gpgkeys
Merge pull request #1450 from JacobCallahan/master

Removing the broken link for gpgkeys
Merge pull request #1443 from sthirugn/sam-ui-fencing

UI sam fencing
Consolidated URLs with constants.

Throughout the code, tests included, we have many variables pointing to
a URL which is in turn used for some of our tests or default values. I
have created several new constants for these URLs and updated the code
accordingly.
Merge pull request #1454 from omaciel/update-urls

Consolidated URLs with constants.
Documentation update for sam fencing with code samples
Merge pull request #1453 from sthirugn/sam-fencing-doc

Documentation update for sam fencing with code samples
Removing broken link from CLI documentation
Chenge docstring to the correct test class
Merge pull request #1456 from JacobCallahan/master

Removing broken link from CLI documentation
(API) Fix up the "OperatingSystem" entity.

Give the `OperatingSystem` entity three new attributes:

* architecture
* medium
* ptable

Override `OperatingSystem.create` so that these attributes can be correctly
specified. Write new API tests for the "architecture" and "ptable" attributes.
Do not create a test for the "medium" attribute, as it is not currently possible
to create a medium using the API tools.