Switch gprd to use gkms instead of chef-vault for the consul cluster
[@ahmadsherif and I noticed](https://gitlab.slack.com/archives/C9W4C89LY/p1522278281000139) while setting up gstg that gprd uses chef-vault to store the consul cluster secrets. Since moving forward our intention seems to be to prefer gkms, should we move to that for these secrets?
/cc @gl-infra
issue