Avoid collision in random member names

[hongchaodeng]

We randomized etcd member names in #1872. However, the key space is very small 27^5 = 14.3M. We should consider collision case.

Potential solutions:

1. Use UUID. We have one: https://github.com/pborman/uuid . Or update it with new one: https://github.com/satori/go.uuid .
2. We probably just have 2 or 3 members. Just check name conflict before creating one. Note that we couldn't avoid "previous cluster" case in this way.
3. Or just extend length to 10, key space becomes 27^10=2e14

[hongchaodeng]

We will go with option (3).
This requires little changes. From my experience, it is very unlikely to conflict under 2e14 key space.