fix ghost.initPlugins: the activePlugins setting wasn't read correctly.

- fix comment: activePlugins is read from settings, not from config.
- model.Settings.activePlugins: this doesn't work and is replaced by model.Settings.read('activePlugins').
- because Settings.read returns a promise, the last line is wrapped with "when(...).then(...)".

The commit is necessary to get plugins working. To test this you can manually change the activePlugins setting in the settings database.