'juju action fetch' should allow output of a single result

[Tim Penhey]

By default, when fetching results for a complete action, the entire result structure is returned, including the status and timing information.

In order to make this more scriptable for some situations, it would be good to be able to specify just a single result that was set by the action script.

Something like:

  juju action fetch <uuid> result.foo

or

  juju action fetch <uuid> result.foo status timing.completed

If there is only one element specified, the default output should not attempt to make it YAML, but just output it (like the behaviour of 'juju api-info')