Minion-specific etcd pillar values

This allows you to specify something such as the following in your
master config to get minion-specific values in etcd that may override
shared values.

    local_etcd:
      host: 127.0.0.1
      port: 4001

    ext_pillar:
      - etcd: local_etcd root=/salt-shared
      - etcd: local_etcd root=/salt-private/%(minion_id)s