Trainer register etcd

[typhoonzero]

Fix #3051

[gongweibao]

Just a comment:
没有单测的话，如果确定是对的？

[typhoonzero]

etcd client暂时没有好的方法单测。只能集成测试或者e2e test，如果有好的测试方法，欢迎PR~

[Yancey1989]

How about write the pass status into etcd as the following?

type TrainerStatus struct {
    trainerIP string
    pass_num int
    pass_status string
}

For the master check a pass status:

1. master check trainer pass status:
   1.1. if all trainer finished a pass, goto 2
   1.2. else goto 1
2. master move task from Done Queue to Todo Queue.

For the trainer check a pass status:

1. fetch task from master
   1.1. if NoTaskFound, Update status to Finished and goto 2
   1.1. else training with the task
2. if pass_num == finished_pass_num; then finish the train; else goto 1.

[helinwang]

@typhoonzero @gongweibao 有了glide之后我们可以用https://godoc.org/github.com/coreos/etcd/embed 了，欢迎PR哈！

[helinwang]

Maybe we don't need this PR yet due to the latest change in #2948 ? If so maybe we can close it, and reopen it once needed :)

[typhoonzero]

Closing, reopen when needed.