Support lists of KVP in kwargs when calling add_new_repo

[anish]

The loop in substitute() returns when substitutions is non-empty. When parsing over a list, this condition is met on the first member of the list and the rest of the list is not parsed and thrown away. This can be easily reproduced by using a repo that uses more than one gpgkey :

    base.repos.add_new_repo('foo_repo', conf,
        baseurl=['https://foo/bar'],
        gpgkey=['https://foo/bar/key1','https://foo/bar/key2'],
        gpgcheck=True)

At the end of of add_new_repo, the new repo will only contain one key and not a list like this code expects

[m-blaha]

Thank you!