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.
According to the documentation, 'N' is not valid, but golly gee, the documentation lies!

After checking the code and unit tests in MCollective and trying it out, I can say with confidence that 'n', 'f', 'false', and all variations in capitalisation on those values are valid.  However, it does need to be some value that is recognised as false: `mco inventory` fails if I leave this setting unspecified or specify an unrecognised string.  I'll update the code accordingly.