Support changing default user via cloud-init

when change default user use cloudinit,
`multipass shel`l (also `multipass exec`) will fail, with message
```
shell failed: ssh failed to authenticate: 'Access denied for 'publickey'. Authentication that can continue: publickey'
```

#### launch
```sh
multipass launch instance use --cloud-init cloud-config.yaml
```

#### cloud-config.yaml
```
system_info:
default_user:
name: admin
lock_passwd: true
groups: users, admin
sudo: ["ALL=(ALL) NOPASSWD:ALL"]
shell: /bin/bash
passwd: $6$xxxxxxe.
```

Start by reproducing the `multipass launch instance use --cloud-init cloud-config.yaml` example with the shown `system_info.default_user` configuration, then inspect how cloud-init user changes interact with SSH setup for `multipass shell` and `multipass exec`. Done means both commands authenticate successfully after the default user is changed.

Written by the indexing model from the issue text.