feat: support to connect etcd with `username` and `password`

[membphis]

etcd can specify access username/password and assign different permissions, which is more secure.

manager-api needs to support this feature.

[membphis]

related PR: #811

[juzhiyuan]

Will this be resolved in 2.2?

[membphis]

I think 2.2 is fine. I add this issue to 2.2 milestone

[juzhiyuan]

Who is glad to resolve this issue?

[nic-chen]

close by #951