microk8s controller nameclash

[Jordan Barrett]

Suppose I have two users `user1` and `user2`, both running Juju, with access to a common cloud. If this is an lxd cloud, then both users can bootstrap a controller with the same name:
```
user1:~$ juju bootstrap lxd lxd
Creating Juju controller "lxd" on lxd/localhost...

user2:~$ juju bootstrap lxd lxd
Creating Juju controller "lxd" on lxd/localhost
```
This is because, behind the scenes, each controller is assigned a random lxd container name such as `juju-0e52f1-0`, so they won't clash.

However, for a microk8s cloud, this is not possible:
```
user1:~$ juju bootstrap microk8s k8s
Creating Juju controller "k8s" on microk8s/localhost...

user2:~$ juju bootstrap microk8s k8s
ERROR a controller called "k8s" already exists on this k8s cluster.
Please bootstrap again and choose a different controller name.
```
This is because the `k8s` controller is assigned the namespace `controller-k8s` and name `controller-0`. As a result, `user2` is unable to create a controller with the same name.

To solve this issue, it should be easy enough to assign a random namespace/pod name to new k8s controllers, so that there is no clashing.

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.