Add cli args for etcd basic auth

[shanegibbs]

Adds support for --etcd-username and --etcd-password args.

[philips]

this seems fine

[dcbw]

Seems fine to me too

[cvle]

+1 Hope this will be merged soon.

[shanegibbs]

@philips, what would the process to get this merged be? Anything I can help with?

[shanegibbs]

Looks like the etcd client v3 won't be using basic auth

[tomdee]

Small, simple and potentially useful change. We'll cross the etcd v3 bridge when we get to it. Merging.