add --app option to juju run-action

[Joe Guo]

Originally, juju run-action can only target on a single unit.
Bug[0] and PR[1] make it possible to run an action on multiple units, which is helpful.

[0]: https://bugs.launchpad.net/juju/+bug/1667213
[1]: https://github.com/juju/juju/pull/7244

However, it would be even better if we can run action on all units of an app.
Just like what `juju run --app` does.

An example scenario[2][3]:

- in nrpe charm, we are adding a reboot check, which will raise alert if a unknown reboot is detected via `uptime --since`
- we also add an action `ack-reboot` to acknowledge the alert, by updating the known reboot time.

This will be ok for occasional machine reboot and meet our requirement.
However, in a power outage scenario, we will need to run the action on all nrpe units to acknowledge the reboot. I would hope we can simply do this with:

    juju run-action --app nrpe ack-reboot

(Or other similar function like discussed in the PR[1])

[2]: https://bugs.launchpad.net/charm-nrpe/+bug/1952787
[3]: https://code.launchpad.net/~guoqiao/charm-nrpe/+git/nrpe-charm/+merge/413385

[Tom Haddon (mthaddon)]

I agree this would be nice. Or a `juju run-action app/all` to go along with the current syntaxt of `juju run-action app/leader`.