Add support for per-config-option comments

[Andrea Ieri]

Charms are opinionated by design, and can in many cases be deployed without modifying their default config. Often config options need to however be modified in response to specific conditions pertaining to that deployment, sometimes even temporarily.

I believe it would be very convenient if any charm config option had comment metadata attached to it, so that the juju db could be the single authoritative source of a cloud design without requiring external tracking.

One could imagine for example performing the following changes:

juju config rabbitmq-cluster modulo-nodes=4 --comment="workaround for charm-helpers/issues/142"
juju config neutron-api enable-igmp-snooping=true --comment="case 123456"

An operator would then be able to analyse the cloud configuration and understand the context or reasoning behind certain configuration choices.

[of course one can add comments to bundles, but those comments are not juju constructs and will be lost upon deployment]

[John A Meinel (jameinel)]

I believe you can, currently, set arbitrary annotations on objects in the juju model (it was added for the GUI to be able to add notes like 'where should this show up'), but those aren't exposed on the CLI, and I do think the syntax listed above is an interesting way of describing it.

Though often config actually comes in via bundles, eg:

applications:
  foo:
    charm: foo
    options:
      enable-igmp-snooping: true

And we would definitely want to include a way to have a comment on that option. (obviously yaml supports comments, but you'd want to preserve, export and import those comments across deploy and export-bundle.)