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
Merge pull request #1676 from Ichimonji10/api-orm

Run tests that were skipped due to BZ 1162860
Partially skip test due to BZ 1163494

Issuing an HTTP GET request for a non-existent repository triggers an HTTP 500
error with this error message:

    NoMethodError: undefined method `[]' for nil:NilClass

This only occurs if a repository with the given ID once existed. Fetching a
repository that has never existed will trigger an HTTP 404 as usual with a
message like this:

    Couldn't find Katello::Repository with id=600

Updated test results:

    $ nosetests tests/foreman/api/test_multiple_paths.py:DoubleCheckTestCase -m 'test_delete_and_get.*Repository'
    .
    ----------------------------------------------------------------------
    Ran 1 test in 11.153s

    OK