Functional tests conflict with existing cluster

The functional tests use the `/_coreos.com/fleet` namespace of the host machine's etcd server. This can cause odd behavior if other fleet services are running on the host. The functional tests also clean up after themselves by blowing away `/_coreos.com/fleet/*`. This is just not nice.

We need to figure out how to completely partition our test fleet cluster off from the rest of the world. Maybe we deploy a separate etcd service for the functional tests, or we could make the etcd namespace configurable.