Add pulp-admin command to list repository units.

Add commands to list ostree units.

Sample output:

```
[jortel@localhost pulp_ostree]$ pulp-admin ostree repo search --repo-id=jeff
+----------------------------------------------------------------------+
                             Content Units
+----------------------------------------------------------------------+

Id:        54465eafe138231f61748822
Created:   2014-10-21T13:25:03Z
Updated:   2014-10-21T13:25:03Z
Remote Id: fe3150788b0d5f396a4834c7be7ec4f83d47fda8b3f9ff8404a789c28d4d22e7
Digest:    e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
Refs:      
  Heads: 

Id:        544673d0e138231f61748824
Created:   2014-10-21T14:55:12Z
Updated:   2014-10-21T14:55:12Z
Remote Id: fe3150788b0d5f396a4834c7be7ec4f83d47fda8b3f9ff8404a789c28d4d22e7
Digest:    7f2343063a9daa18b062507980da0b3718a28bf3b31d1286e8ef8655821f42f7
Refs:      
  Heads: 
    Commit Id: 005d4caafc862853b920b9a0ab2f5af2b197cc13024a49a6add85bae4a8a40ee
    Path:      fedora-atomic/f21/x86_64/docker-host


```
I think the term "filter" might be misleading here. I assume that means some units might be removed from the collection, but that does not seem to be the case. This is just doing a transformation on each item.

Given the workflow, you might consider re-arranging this function so that it transforms just one item, and calling ``map(self.transform_unit, units)`` on line 105 below or using a list comprehension such as ``[self.transform_unit(u) for u in units]``. It's obviously not a big deal to do the iteration yourself, but there will be slightly less code and slightly less complexity by letting a built-in tool manage the iteration.

That pattern would also lend itself to making a generator, such as with ``itertools.imap``.