oo-diagnostics: test_mcollective_direct_addressing

Add `test_mcollective_direct_addressing`, which checks that `direct_addressing` is disabled in MCollective's `client.cfg` (on broker hosts) and `server.cfg` (on node hosts).

This commit fixes bug 1144615.
Even though probably nobody runs oo-diag outside broker/node, I sort of like having this up front to be clear about the intention. I don't feel strongly either way though.