Description of problem:
    running pulp-admin repo sync against a repository which has removed/replaced content raises an exception and stops cold when preserve_metadata is set to 'True'.

Version-Release number of selected component (if applicable):
    Community Release 17
    Community Release 18

How reproducible:
    Perfectly

Steps to Reproduce:
1. Sync a repository which removes/replaces content with the perserve_metadata flag set to true
2. Wait until repository has replaced or removed content
3. Try to sync the repository once more.
  
Actual results:

At the command line:
PulpException: Metadata for repo [repository-id] is set to be preserved. Cannot remove new content

In /var/log/pulp/pulp.log
2011-11-08 14:21:25,208 13173:140249355228928: pulp.server.api.synchronizers:INFO: synchronizers:275 Skipping errata imports from sync process
2011-11-08 14:21:25,212 13173:140249355228928: pulp.server.api.repo_sync:INFO: repo_sync:263 Sync returned 81 packages, 0 errata in 20.8483200073 seconds
2011-11-08 14:21:30,030 13173:140249355228928: pulp.server.api.repo_sync:INFO: repo_sync:274 81 old packages to process, 81 new packages to process
2011-11-08 14:21:30,034 13173:140249355228928: pulp.server.api.repo:INFO: repo:956 Metadata for repo [redhat-cdh-3] is set to be preserved. Cannot remove new content
2011-11-08 14:21:30,039 13173:140249355228928: pulp.server.tasking.task:ERROR: task:455 Task failed: Task cd19d18a-0a3e-11e1-a635-00137254a118: _sync(redhat-cdh-3, synchronizer=<pulp.server.api.synchronizers.YumSynchronizer object at 0x7f8e95f01e10>, skip_dict={}, max_speed=None, threads=None, progress_callback=<bound method RepoSyncTask.progress_callback of <pulp.server.api.repo_sync_task.RepoSyncTask object at 0x7f8e95f01590>>)
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pulp/server/tasking/task.py", line 404, in run
    result = self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo_sync.py", line 277, in _sync
    repo_api.remove_packages(repo["id"], old_pkgs)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo.py", line 957, in remove_packages
    raise PulpException(msg)
PulpException: Metadata for repo [redhat-cdh-3] is set to be preserved. Cannot remove new content

Expected results:
Yo! found you new rpms! Toss them dirty old ones out.

Additional info: