Added model-default as alias to model-defaults.

[anastasiamac]

Description of change

For usability, this PR adds an alias to 'model-defaults' command - 'model-default', singular.

QA steps

1. 'juju help commands' lists 'model-default' as well.
2. running 'juju model-default <model-config-param=param-value>' has the same effect as running 'juju model-defaults <model-config-param=param-value>'.

Documentation changes

A command alias has been added.

Bug reference

https://bugs.launchpad.net/juju/+bug/1656216

[anastasiamac]

$$merge$$

[jujubot]

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

[anastasiamac]

@pmatulis,

This change may require documentation update.

[pmatulis]

@anastasiamac The commands.md page needs updating but that's a different matter. Other than that, the docs don't make any mention of command aliases (with the exception of a single page, actions.md). I think this is a wise approach (prevents clutter and cuts down on confusion).