fleetctl: wait for unit creation to prevent spurious 404 errors
fleetctl: wait for unit creation to prevent spurious 404 errors

in testing with coretest, fleetd would get spurious errors while
creating and then reading back units. it is a race that occurs when a
PUT (consistent write to etcd) is immediately followed by a GET
(inconsistent read) to the same unit.

the real fix for this is to make use of etcd indexes everywhere in
fleetd, but unfortunately etcd 0.4.x indexes are broken. as a
workaround, we block in fleetctl waiting for acknowledgement of unit
creation before proceeding to load/start/etc.

improbable-io/fleet,coreos/fleet,octoblu/fleet,esatterwhite/fleet,bviswana101/fleet,WPMedia/fleet,mookjp/fleet,WPMedia/fleet,paveltiunov/fleet,dalbani/fleet,thepwagner/fleet,thepwagner/fleet,paveltiunov/fleet,endocode/fleet,octoblu/fleet,endocode/fleet,giantswarm/fleet,dongsupark/fleet,improbable-io/fleet,coreos/fleet,jonboulle/fleet,endocode/fleet,dalbani/fleet,dongsupark/fleet,coreos/fleet,dalbani/fleet,flandr/fleet,mookjp/fleet,jonboulle/fleet,improbable-io/fleet,giantswarm/fleet,jonboulle/fleet,bviswana101/fleet,dongsupark/fleet,flandr/fleet,esatterwhite/fleet,giantswarm/fleet