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
Merge pull request #1212 from deis/fix_etcd_safe_set

fix(*): fix etcd_safe_set to not override values