[election2] Add etcd client creation wrapper (#1289)
[election2] Add etcd client creation wrapper (#1289)

NewClient helps external repos create etcd Client from this repo's
vendored package, and use it with the election library, without
explicitly importing trillian/vendor/.../etcd (if that is at all possible).
AlCutter/trillian,google/trillian,RJPercival/trillian,gdbelvin/trillian,Martin2112/trillian,google/trillian,RJPercival/trillian,daviddrysdale/trillian,gdbelvin/trillian,Martin2112/trillian,daviddrysdale/trillian,gdbelvin/trillian,AlCutter/trillian,Martin2112/trillian,daviddrysdale/trillian