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