config.changed trigger not triggering

[stub42]

I have a trigger on config.changed, which is not firing:

reactive.register_trigger(when='config.changed', clear_flag='postgresql.configured')

I think this is because caas_base is initializing the flags on module import, rather than scheduling it for later using hookenv.atstart like the base layer does. At this point, none of the triggers are registered as none of the code in reactive/ has been discovered.

[johnsca]

Rather than porting canonical/layer-basic#126 here and hoping it doesn't get out of sync again, maybe we should go ahead and move forward with pulling the flags into charms.reactive.