fix(app-admin/fleet): Add service dependency on etcd.

Fleet without etcd isn't particularly useful. Make sure it starts to
avoid the usual surprises from new users. No need for strict ordering.
fix(dev-db/etcd): Re-add [Install] section to service file.

This was needlessly removed in 03be5aef, required for configuring etcd
to start via `systemctl enable etcd.service`.
Merge pull request #508 from marineam/units

Updates for etcd and fleet units