fix(etcd_test): use strong consistency
fix(etcd_test): use strong consistency

Or it may fail easily on follower machines.

quantum/coreos-mantle,dm0-/mantle,quantum/coreos-mantle,marineam/mantle,mischief/mantle,dm0-/mantle,glevand/coreos--mantle,mjg59/mantle,marineam/mantle,glevand/coreos--mantle,mischief/mantle,mjg59/mantle