rework etcd3 install to be like other devstack services

This separates out the install phase early from the start phase to
make this mirror other services in devstack.

Depends-On: I4124dc7e3fd3b4d973979da85209ec991c0f8c4b

Change-Id: I76f8740448b25a48869ee80006e826baa6cafc2b
diff --git a/stack.sh b/stack.sh
index cfdc2c4c..c468595 100755
--- a/stack.sh
+++ b/stack.sh
@@ -813,6 +813,10 @@
     install_neutron_agent_packages
 fi
 
+if is_service_enabled etcd3; then
+    install_etcd3
+fi
+
 # Check Out and Install Source
 # ----------------------------