etcdserver: have newMember take an optional time field

This will be used by members joining an existing cluster or joining
using discovery.
etcdserver: stop using addScheme

This standardizes the flags to use a list of URLs everywhere. The next
step is to enforce the scheme based on TLS settings and support compat
flags.
Merge pull request #1213 from philips/improve-cluster

etcdserver: stop using addScheme