Add a function pkg.list_repo_pkgs

List the packages from repo
```
shell# salt  'node1'  pkg.list_repo_pkgs
node1:                                                                                                                                                                  
    ----------
    repo:
        ----------
       epel:
            - 389-ds-base-devel-1.2.11.15-11.el6.i686.rpm
```
https://github.com/itnihao/salt/commit/109732b2edb097818fb158b8359a5d17df324b5b