RBAC should validate the service name for `content_type`

[AlanCoding]

The /role_definitions/ list accepts content_type as one of the fields in the format of "aap.organization". The intent is that the first prefix is the service name, for a cluster of multiple services stitched together by the JWT and resource_registry communication.

While that's the intent, the code just ignored it, test case:

def test_service_name_is_validated(admin_api_client):
    url = get_relative_url('roledefinition-list')
    response = admin_api_client.post(
        url,
        data={
            'name': 'Random custom role',
            'content_type': 'justsomerandomstuff.organization',
            'permissions': ['aap.view_organization', 'local.change_organization'],
        },
    )
    assert response.status_code == 400, response.data
    assert 'service name not valid for model organization' in str(response.data)

This fails with a 201 response_code.

We already had a requirement that the model name is unique within the permission registry. This is intended to change later, to be unique within the registry for a particular service... specifically to support awx.credential and eda.credential, which is a legitimate problem.

However, since we don't have any non-synced services now, model name is safely unique by itself right now, so writing the serializers, I just stopped where I was. This is a TODO for later, as we develop a permissions-provider functionality.