juju wait-for model $model hangs if $model not current

`juju wait-for model $model` only returns if $model is the current model. Otherwise you need to specify `-m $model`. An example session:

```
$ juju models
Controller: test

Model Cloud/Region Type Status Machines Cores Units Access Last connection
one* aws/ap-southeast-2 ec2 available 7 14 11 admin 30 seconds ago
two aws/ap-southeast-2 ec2 available 2 4 5 admin 5 minutes ago

#(models one and two are all active/idle)

$ juju wait-for model one
Model "one" is running
properties:
  life: alive
  status: available

$ juju wait-for model -m two
ERROR model name must be supplied when waiting for an model

$ juju wait-for model -m two two
Model "two" is running
properties:
  life: alive
  status: available

$ juju wait-for model two
^C
#(hangs)

$ juju switch two
test:admin/one -> test:admin/two

$ juju wait-for model two
Model "two" is running
properties:
  life: alive
  status: available
```