Migration to the new Etcd client

This commit migrates the old 'go-etcd' client, which is deprecated
to the new 'coreos/etcd/client'.

One notable change is the ability to specify an 'IsDir' parameter
to the 'Put' call. This allows to circumvent the limitations of etcd
regarding the key/directory distinction while setting up Watches on
a directory. A conservative measure to set up a watch that should be
used the same way for etcd/consul/zookeeper is to enforce the 'IsDir'
parameter with 'WriteOptions' on 'Put' to avoid the 'NotANode' error
thrown by etcd on Watch call. Consul and zookeeper are not using the
'IsDir' parameter.

Signed-off-by: Alexandre Beslic <abronan@docker.com>