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

Author: carmstrong

bin/boot

@@ -24,17 +24,15 @@ done
 # wait until etcd has discarded potentially stale values
 sleep $(($ETCD_TTL+1))
 
-function etcd_safe_set {
-  if ! etcdctl --no-sync -C $ETCD get $ETCD_PATH/$1 >/dev/null 2>&1; then
-    etcdctl --no-sync -C $ETCD set $ETCD_PATH/$1 $2 >/dev/null
-  fi
+function etcd_set_default {
+  etcdctl --no-sync -C $ETCD mk $ETCD_PATH/$1 $2 >/dev/null 2>&1 || true
 }
 
-etcd_safe_set protocol ${DEIS_PROTOCOL:-http}
-etcd_safe_set secretKey ${DEIS_SECRET_KEY:-`openssl rand -base64 64 | tr -d '\n'`}
-etcd_safe_set builderKey ${DEIS_BUILDER_KEY:-`openssl rand -base64 64 | tr -d '\n'`}
-etcd_safe_set registrationEnabled 1
-etcd_safe_set webEnabled 0
+etcd_set_default protocol ${DEIS_PROTOCOL:-http}
+etcd_set_default secretKey ${DEIS_SECRET_KEY:-`openssl rand -base64 64 | tr -d '\n'`}
+etcd_set_default builderKey ${DEIS_BUILDER_KEY:-`openssl rand -base64 64 | tr -d '\n'`}
+etcd_set_default registrationEnabled 1
+etcd_set_default webEnabled 0
 
 # wait for confd to run once and install initial templates
 until confd -onetime -node $ETCD -config-file /app/confd.toml 2>/dev/null; do