Renaming "default" to "is_default" in the application get output.

[anastasiamac]

Description of change

Application configuration output was subject to interpretation when displaying default values.
This PR renames "default" to "is_default" to avoid confusion.

QA steps

1. Bootstrap
2. Deploy application with some config setting modified
3. run 'juju config' for this application
4. Default values, inherited or equal to charm defined settings, should be described as "is_default: true" instead of "default: true"

Documentation changes

If the output sample for 'juju config' contains output for default values, the output needs to be updated.

Bug reference

https://bugs.launchpad.net/juju/+bug/1709692

[anastasiamac]

$$merge$$

[jujubot]

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

[jameinel]

Thoughts on this...
This changes what the controller returns, but not what the client displays. Which means that 2.3 against a 2.2 controller still reports "default" while 2.2 against a 2.3 controller would display "is_default".
I'm a little concerned about it, as this is the controller changing what the client displays.

There is precedent, as its what 'juju status' does.

I only noticed while verifying the fix, and found that I wasn't seeing the fix when switching clients.

We could make this client specific if we versioned the API, and had 2.3 clients know that when talking to the old API they translate the field, and controllers know to use the right field when clients ask for the new API version.

Its probably not worth the effort, but I do wonder...