Add extra-info as official string config value for models.

[howbazaar]

This work adds a model config value 'extra-info' as an official string value to allow users to store arbitrary data against a model. This has been requested by conjure-up.

QA steps

$ juju model-config extra-info="---\nname: this is more yaml\n"
$ juju model-config extra-info
'---\nname: this is more yaml\n'

Bug reference

Fixes lp:1659444.

[babbageclunk]

This seems good to me. I'm not sure about making it a string though - wouldn't a map[string]interface{} be a bit more future-proof? Is it just the complexity?

[jameinel]

We have "annotations" on applications for things like GUI and deployer to store their extra information. I wonder if calling it something similar would help.

[howbazaar]

@jameinel we have no CLI way of setting or getting annotations.

I do think that perhaps that would be the future way of doing things where we namespace more.
Pretty sure that we don't migrate annotations for models as there haven't been any as yet.

[howbazaar]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10118

[howbazaar]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju