fix(scheduler/coreos): skip announcer containers for start, stop, destroy

With #1269 we stopped creating announce containers except for web and cmd
types, but we still attempt to perform actions on them for start, stop,
and destroy. This commit adds logic to wrap all of these scheduler actions
in a conditional which skips (and logs) announce operations.

TESTING: rebuild the controller
```console
$ make -C controller stop build start
```

Then, deploy an application which has either a cmd or web proctype, as
well as a non-standard type. I modified the Procfile for our
example-ruby-sinatra like so:

```
web: bundle exec ruby web.rb -p ${PORT:-5000}
clock: echo flava flavvvvvvv
```

Scale the app, start/stop processes, etc. You should see successful
operations for both, and note that the clock type has no announce
containers. The output of ```deis ps``` should always show them
as being up:

```
=== exotic-icehouse Processes

--- web:
web.1 up (v3)
web.2 up (v3)
web.3 up (v3)

--- clock:
clock.1 up (v3)
clock.2 up (v3)
clock.3 up (v3)
```

fixes #1273
closes #1275
This is an explicit property declaration, not through the @property decorator, although I don't think it makes much difference. I agree in principle with your comment about the intent of python properties, but I think it's more of a personal preference than a hard rule--attribute access can be more "pythonic" than method calls in some cases. Either way I don't think we should hold up this customer-driven PR over it.