Add option to default to disabling all plugins

We currently enable all plugins by default (with the exception of those marked experimental). This will have the side effect of automatically adding new plugins during upgrades, and assumes certain things about a deployment (like it's using Neutron).

It's been suggested instead to make 'enabled' an inheritable plugin option (available on [resource_plugin]) with a note explaining that a deployment can be 'opt-in' (requiring enabled=true for supported plugins) or 'opt-out' (the current behavior). Whether the default is opt-in or opt-out is up for debate; requiring explicit opt-in is perhaps safer. This will also benefit testing where a single plugin is under test.