host: Ensure etcd is started with a unique name

When using the libvirt-lxc backend, it is likely that HOSTNAME will not
be set, resulting in etcd always being started with a name of "<no
value>" which is problematic for multi node clusters.

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #144 from flynn/unique-etcd-name

host: Ensure etcd is started with a unique name