Run tests that were skipped due to BZ 1162860

BZ 1162860 states that it is impossible to interact with a server using any user
other than the admin user. This is a false alarm, and the test errors occurred
due to an issue in robottelo itself. Drop references to BZ 1162860 and fix the
issue.

The `Field` class can store the "default" and "choices" instance attributes.
Unfortunately, `OneToOneField.get_value` and `OneToManyField.get_value` ignore
these attributes. Fix this issue by simplifying all `*Field.get_value` methods,
dropping `robottelo.orm._get_value`, and moving some extra value generation
logic in to `robottelo.orm.EntityCreateMixin.create_missing`. Drop some tests
to compensate for this change.

The test suite still behaves nicely. The four errors occurred because
manifest-related settings were not set, and the four failures are existing
issues.

    $ make test-foreman-api
    nosetests -c robottelo.properties tests/foreman/api
    Ran 773 tests in 3204.702s

    FAILED (SKIP=102, errors=4, failures=4)
    Makefile:41: recipe for target 'test-foreman-api' failed
    make: *** [test-foreman-api] Error 1