AS-394: Filter incorrect groups

Adding `status` filed in `scaling_groups` that will be used to filter out groups that are incorrect and are skewing our metrics. This is also the first step towards providing group status to user when the group cannot be scaled due to invalid launch configuration. 
This is pretty cool!  Like `all` but lazy.  Think this might be worth a PR to `toolz`?  I'm surprised it's not in there given how useful it is.