RPM Support

Oracle 8 Appstream repo (http://yum.oracle.com/repo/OracleLinux/OL8/appstream/x86_64/) miss "arch" in few modulemds.

  "error": {
    "traceback": "  File \"/usr/local/lib/pulp/lib64/python3.8/site-packages/rq/worker.py\", line 883, in perform_job\n    rv = job.perform()\n  File \"/usr/local/lib/pulp/lib64/python3.8/site-packages/rq/job.py\", line 657, in perform\n    self._result = self._execute()\n  File \"/usr/local/lib/pulp/lib64/python3.8/site-packages/rq/job.py\", line 663, in _execute\n    return self.func(*self.args, **self.kwargs)\n  File \"/home/vagrant/devel/pulp_rpm/pulp_rpm/app/tasks/synchronizing.py\", line 208, in synchronize\n    dv.create()\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/declarative_version.py\", line 148, in create\n    loop.run_until_complete(pipeline)\n  File \"/usr/lib64/python3.8/asyncio/base_events.py\", line 616, in run_until_complete\n    return future.result()\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/api.py\", line 225, in create_pipeline\n    await asyncio.gather(*futures)\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/api.py\", line 43, in __call__\n    await self.run()\n  File \"/home/vagrant/devel/pulpcore/pulpcore/plugin/stages/content_stages.py\", line 104, in run\n    d_content.content = d_content.content.__class__.objects.get(\n  File \"/usr/local/lib/pulp/lib64/python3.8/site-packages/django/db/models/manager.py\", line 82, in manager_method\n    return getattr(self.get_queryset(), name)(*args, **kwargs)\n  File \"/usr/local/lib/pulp/lib64/python3.8/site-packages/django/db/models/query.py\", line 406, in get\n    raise self.model.DoesNotExist(\n",
    "description": "Modulemd matching query does not exist."
  },

Proposed solution: fix only in sync time as by ModuleMD spec file this should be mandatory.