fleetctl: Remove unit content from etcd
fleetctl: Remove unit content from etcd

Fleetctl destroy unit does not remove the unit file content from
etcd registry(/_coreos.com/fleet/unit/XXXX). It will cause so
many junk to leave in the etcd. The modification removes it from
etcd when destroying unit.

Fixes #1290

wuqixuan/fleet,wuqixuan/fleet