Fix #106

[bcwaldon]

% fleetctl submit build/units/subgun.1.service
Creation of payload subgun.1.service failed: 105: Key already exists (/_coreos.com/fleet/payload/subgun.1.service) [2538]
% fleetctl start build/units/subgun.1.service
Creation of job subgun.1.service failed: 105: Key already exists (/_coreos.com/fleet/job/subgun.1.service/object) [2542]
% fleetctl --endpoint=http://127.0.0.2:3001/ start build/units/subgun.1.service
Creation of payload subgun.1.service failed: 501: All the given peers are not reachable (Tried to connect to each peer twice and failed) [0]

[philips]

lgtm

[philips]

/cc @unihorn you can review code in this repo too.

[philips]

@unihorn Just to get you slightly familiar with a client of etcd.