Metadata: add some methods to manage clients

This adds list_clients and remove_client as rmi methods, so that it could be called with bcfg2-admin xcmd.
Unfortunately, NoneType no longer exists in python3 (see http://bugs.python.org/issue19438). One alternate solution which works for me is: https://gist.github.com/solj/9cc286ab47c2cac317e2