Geo pg-password-md5 not supported in 10.3

@xiaogang_gitlab and a customer attempted to run gitlab-ctl pg-password-md5 gitlab via https://docs.gitlab.com/ee/gitlab-geo/database.html, but this fails in 10.3.

Until we have versioned documentation, I think we should mark this a 10.4+ command?

Edited Dec 29, 2017 by Stan Hu