etcdctl: add help on the members subcommands
etcdctl: take a name and print out the initial cluster

To help the user lets print out the configuration that they will need to give
to their new member:

$ etcdctl member add infra4 http://localhost:7004
added member 9bf1b35fc7761a23 to cluster
ETCD_NAME="infra4"
ETCD_INITIAL_CLUSTER="node2=http://localhost:7002,node3=http://localhost:7003,infra4=http://localhost:7004,node1=http://localhost:7001"

This is a little weird because the API doesn't take a name so the user gives us
a name and we just pass it on through.
Merge pull request #1568 from philips/dynamic-configuration

etcdctl: two member fixes