Add etcd as a submodule so we can use its raft implementation.

This commit adds the dependency without really using it so we can make
sure that the submodule process works for everyone without causing
problems. Everyone will need to `git submodule update --init`
after pulling this commit.

Note that we use the import path 'coreos/etcd' but create the submodule
from 'cockroachdb/etcd'. This allows us to maintain and use our own
changes without waiting for everything to be merged upstream by etcd.
We keep the import path because otherwise raft internally uses multiple
packages and we don't want to rewrite its internal imports.