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
3 files changed