Remove "admin" from /v2/admin/members

The "admin" prefix isn't really necessary, the "members" API is just another API supported by etcd - same as "stats" and at a later date, "config". 
I'm all for it. There's the APi people care about `/v2/keys` then all the other APIs that matter to a subset of users `/v2/members`, `/v2/stats`, `/v2/config`.