fix(services): prevent units from stopping as "failed/failed"

Deis' systemd units always return "failed/failed" after being stopped
by fleetctl. Somehow our ExecStop command to remove the service container
conflicts with our ExecStart command and causes it to return an error code.
This is fixed by removing the ExecStop commands and adding a `--rm` flag
to the docker command in ExecStart. Additionally, our on_exit() handler in
each /app/bin/boot script was not returning 0.

TESTING: Run `make run`, then `make stop` on a Deis cluster. You should see
all units stop and return to "inactive/dead" status, whereas before they
would show "failed/failed."
Merge pull request #1224 from deis/1222-make-run-fix

fix(services): prevent units from stopping as "failed/failed"
fix(*): fix etcd_safe_set to not override values

Refactors etcd_safe_set so it doesn't overwrite custom settings.

TESTING: rebuild and restart components, setting a non-default
custom value
```console
dev $ make uninstall
dev $ vagrant rsync && make build
dev $ vagrant ssh
coreos $ etcdctl set /deis/controller/webEnabled 1
dev $ make run
```
Make sure the settings have taken effect.

Changes proposed by @aledbf

fixes #1210