worker/uniter: simplify EnsureTools and make it concurrent-safe
If someone breaks the symlinks, the user "gets everything they deserve".
Avoiding the link reading logic makes it trivial for EnsureTools
to be idempotent, even when run concurrently.
Also fix the agent name to match our upstart convention
which prefixes the tools directory with the agent kind.
R=fwereade, niemeyer
CC=
https://codereview.appspot.com/6461061