Added summary flag for repository lists

Example output:

```
$ pulp-admin rpm repo list --summary
pulp-f19    pulp-f19
errata_demo Errata Upload Testing Repository
upload-test upload-test
f19         Fedora 19
pulp-f19-64 Pulp Fedora 19 x86_64
f19-updates Fedora 19 Updates
empty-rpm   Empty Repository (forget what this is for)
```