Better startup error handling and configurability

Short circuit kubelet initialization if the root volume directory can't be
created, as volume mounts are non-functional in the absence of the directory.
Without this check, volume mounts (including Docker socket mounts for builds)
fail in non-obvious ways.

Make the etcd and volume directories configurable to better facilitate dev
and test environments. Retain the existing defaults for turnkey prod-like
operation.
The etcd store and user volumes directories will have different requirements and I don't think they should  be co-located.