Lets Encrypt Staging config points to obsolete server

Problem/Motivation

When we try to install a site with HTTPS enabled, with our server using the "Staging" Lets Encrypt server, we see the following in the output:

# INFO: Using main config file /var/aegir/config/letsencrypt/config.staging
ERROR: Problem connecting to server (get for https://acme-staging.api.letsencrypt.org/directory; curl returned with 6)
Failed to generate Let's Encrypt certificates.

Further, when we curl that API endpoint, we see it does not resolve:

$ curl https://acme-staging.api.letsencrypt.org/directory
curl: (6) Could not resolve host: acme-staging.api.letsencrypt.org

Reviewing the Lets Encrypt documentation, we see that for Staging, they indicate the correct URL is now: https://acme-staging-v02.api.letsencrypt.org/directory

Proposed resolution

Correct the submodules/letsencrypt/drush/bin/config.staging file in hosting_https that contains the staging URL.

Note: this issue only impacts hosting_https's staging config; the dehydrated script included in hosting_https already has the correct production server.