provide an easy way of changing leader of a certain application

[gabrielcocenza]

On the charm that I've been working, which is subordinate one, it would be nice to have some functional tests that check that nrpe configuration are present just in the leader unit and that this config disappears if leader unit changes. This is what I tried so far:

1. running systemctl stop jujud-machine- on the leader unit by using zaza.model.run_on_unit with a timeout. When I do that some weird behaviors happen:

• timeout is not respected and the command hangs forever. Probably this is due python libjuju depending on jujud-machine being up and running.

2. deleting a unit. Because it's a subordinate charm it's not possible doing that. All subordinates charms would be destroyed if the relation with the principal is removed.

I guess if zaza provides an easy way of changing leader of an application, that could be useful and helpful for a lot of functional tests on charms.