Trim down ForemanTask, test ForemanTask API

Remove the custom method `ForemanTask.read`. Make `ForemanTask` inherit from
`EntityReadMixin`. `ForemanTask.read_json` performs the same task that the old
`ForemanTask.read` method did, and no code was using the old method.

Add a test targeting bugzilla bug 1131702. The test asserts that when a
non-existent foreman task is read, an HTTP 4XX or 5XX error is received. The
test fails, so it is skipped appropriately.

Add this new test module to the API docs.
Use testimony tag `@bz: 1131702`