Support username and password authentication with etcdv3 backend

[rg2011]

What would you like to be added:

Support username and password authentication with ETCD backend

Why is this needed:

ETCD supports username and password authentication since version 2.1, besides of (or on top of) certificates. Usernames and passwords are easier to manage than client certificates, so I think this would help users who want to connect stolon to an etcd cluster that has authentication enabled.