ocg: simplify module hierarchy

There is no reason for "ocg::ganglia::module", "ocg::ganglia" should be
enough. Similarly for Nagios, with the extra catch that there is no need
for two separate classes, one of which is only used by the other and
only contains a single resource.

Change-Id: Ia5c16bfc20d7e9e10cd6860a29a06a65506335b2