Information and format of Couchbase Server Email Alerts should be improved

There are many things to improve about Couchbase Server alerting. This ticket focuses just on the information in the email and its presentation / format.

This is a non-comprehensive list of improvements:

• The email should have a display name that shows it to come from couchbase@[node-name]. (Where node-name is the OTP name of the host with the leading "n_1@" removed.)
  • Note that many SMTP relays require the from address to be a real, verified email address. When I say the display name I mean the part of the email address before the angle brackets. Something like this: couchbase@db1.cb.example.com <cb.alerts@example.com>
• The subject should include the node name and the cluster name. E.g. it should be something like:
  • Couchbase Server alert: approaching full disk warning on node: [node-name] in cluster: [cluster-name]
• We should look at the body text of the email and consider changing the way it presents itself and potentially including more information.
• We should consider sending a HTML mail and a text mail in the MIME body with the idea that the HTML mail is for humans and the text body is for automated alert processing.

A change like this needs to be handled carefully and may only be feasible in a major release as we may break automation that folks have built around alert emails.