[proxy] storage: storage fields overspecified

[Allele Dev]

In the marconi/proxy/storage implementation for mongodb, the field specifiers for query results are unnecessary specific. A fix to this bug would omit what wasn't necessary using the strategy given below.

Schema fields overspecified - use {'_id': 0} instead. Example of current state:

```python
fields = {'p': 1, 'q': 1, 'n': 1, 'h': 1, 'm': 1, '_id': 0}
```

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/50305
Committed: http://github.com/openstack/marconi/commit/cb408d8d01757ab8929fb007db41c1f76d2cbe47
Submitter: Jenkins
Branch: master

commit cb408d8d01757ab8929fb007db41c1f76d2cbe47
Author: Alejandro Cabrera <email address hidden>
Date: Tue Oct 8 07:58:12 2013 -0400

    fix: proxy mongodb storage fields overspecified

    It's unnecessary to specify all the fields that should be returned by
    a mongodb query, because by default, it will return all the
    fields. This patch makes it so that only the '_id' field is mentioned
    specifically, and that's because we don't want it leaking into the
    transport layer.

    Change-Id: Ib59e551b0c85f2972de86baeaa2c7e65738a9a47
    Closes-Bug: 1230390