feat(units): Add generic cloudinit path unit

Switch to triggering common user configs via a path unit. This is
particularly useful for config drive so that a config drive can be
mounted by something other than the udev triggered services, a bind
mount when running in a container for example.
fix(etcd): fix runtime panic when etcd section is missing.

The etcd code tries to assign ee["name"] even when the map was never
defined and assigning to an uninitialized map causes a panic.
Merge pull request #119 from marineam/container

container and panic fixes