juju wait-for: allow for specifying leader units

[Heitor]

Many Juju commands for units can use the format `unit-foo/leader` when we don't know or it is not convenient to use the unit number.

But the Juju plugin wait-for only accepts the unit number:

```
$ juju wait-for unit slurmctld/leader --query='agent-status=="idle"'
ERROR "slurmctld/leader" is not valid unit name
```

I'd like to be able to use the unit-foo/leader notation also in this plugin.

We have an extensive integration test suite that uses the wait-for plugin, but we need to hardcode some unit numbers making local development/testing woobly.